The permission to shorten the prayer, in some specific conditions, is the grace of Almighty but it is misused by the Muslims community. Quran is very clear over the matter but what was the reason to make it complex I could not understand clearly. As I see the reason of complication is to prefer the sayings of persons (self + others) instead of Almighty. The examples are the contrary views of "Ulamaas" that make the matter intricate.

Let see some quoting presented in Tafseer Ibn-a-Kaseer by Ibn-a-Kaseer and Tafheem ul Quran by Mao'doodi sb by the References of Misc. Ahadith books.

Allama Mao'doodi quote in Tafheem ul Quran that
according to Abu Hanifa and Imam Malik (ra) Qasar
is "Waajib", but many others like Imam Shaafi, Imam
Ahmed (ra) said it's not necessary but leave upon
your will.

It's a big difference between which could not be ignored. Some are saying it is necessary but some others are denying. Another example is here

Abu Hanif, Soori, Dawood (ra) said only "travel" is the condition for
Qasar may you are going for robbery. But ibn_a_kaseer said "Jamhoor"
are not agreed with it. As Allama Mao'doodi quote in Taf'heem ul Quran
"according to some most senior persons like Ibn-a-Masood, Ibn-a-Umer,
Atta, Imam Malik there is a condition applied for Qaser that travel should
be for the sake of Allah, otherwise Qaser is not allowed." Or Imam shaafi
and Imam Ahmed (ra) said travel should be only for legal or admit-table
purpose, Qaser is not allowed when you are traveling for an illegal purpose.

The difference is very clear. Let see the matter in another way

Yaa'la bin umai'ya asked Umer (rt) this order is for fear only not
common, Umer (rt) replied I also put this question in front of
Muhammad (saw), He (saw) said this is the grace of Allah so accept
it. (Muslim, Musnad Ahmed)

Abu khanzla (rt) asked Umer (rt) what is travel-salat? Umer (rt) replied
"Two Rakah", Abu khanzla said but this is for fear condition, Umer (rt)
puss this is Sunnah.

According to Ibn-a-abbas we offered only two Rakah mostly when we
travel between Makkah & Madina with Muhammad (saw). (Nisai)

Some other books have nearly same Rivayaat.

The above Rivayaat are saying in clear that to shorten the prayer in travel (for any purpose) is Qasar and it is two Rakah. Muhammad (saw) said it is a grace of Almighty and according to Umer (rt) it is Sunnah, but there are some other Rivayaat that presenting opposite story.

Abdullah bin Umer (rt) said mostly we offered two Rakah when I travel
with Abu Bakar, Umer, Usman (rt) but Usman (rt) started to offer total
Rakah in his last years. (Bukhari)

When it is a grace of Almighty and a Sunnah of Muhammad (saw) then why and how Usman (rt) left it.

Zuhaak, Mujaahid, Suddi and others of same school of thought said
there is no decreasing in the number of Rakah but in "Qiraat" &
"Rakoo-o- Sajood". They present a hadith, narrated by Imam Malik,
in favour of this concept,

Ayesha (rt) said Salat was made essential with two Rakah, afterward
two remained in travel but added two in case of "Iqaamah".

They said when two Rakah is complete Salat then how we could decrease the Rakah. There is another hadith in misc. books that makes strong this concept.

Umer (rt) said, according to Muhammad (saw) travel-salat, Eid ul Fitter,
Eid uz Zuha, Jummah have two Rakah and this is complete prayer.

(Musnad Ahmed, Nisai, Ibn-a-Maaja, Ibn-a-Habbaan)

According to above three quoting two Rakah are Qasar, this is the grace of Almighty and a Sunnah and Waajib. Means it was a regular practice of Muhammad (saw) even then companions of Muhammad (saw) (like Umer) were not aware of this thing, they were hesitated about so asked Muhammad (saw), and it is most surprising. And second three quoting are saying two Rakah is complete prayer in case you are traveling. In first Rivaayat Umer (rt) narrating Muhammad (saw) said two Rakah is Qasar but this is also Umer (rt), in last Rivaayat, narrating that Muhammad (saw) said two Rakah is complete prayer. Here we also got a new aspect that initially Salah was two Rakah, afterward it was amended in four Rakah. When, where and why this was happening there is not any strong evidence.

Let see some other Rivayaat of same pattern

Abdullah bin Abbas (rt) said, according to Muhammad (saw) there are
4 Rakah in case of "Iqaamah", two in travel and one in fear condition.
(Muslim)

Four Rakah in case of Iqaama is also a new thing; we offer 2 Rakah in 1st prayer and 3 Rakah in 4rth salah.

Ibn-a-Jareer narrated that Summaak (rt) asked Muhammad (saw)
about Qaser, He (saw) replied travel-salat has two Rakah and its
complete. Qaser is only in fear condition; and in this condition
Imam recite one Rakah for half of the present and one for other half.

According to above Rivayaat travel-salat consist of two Rakah and it is complete prayer. Qasar is allowed in case you are traveling and your life is in danger and it would be one Rakah. Without fear Qasar is not allowed.

Suddi said travel-salat consists of two Rakah and it's complete.
Qaser is when you have the fear of enemy, shorten the prayer is
not "Halaal" without fear.

Let see the matter another point of view

Ibn-a-Abbas, Imam Malik, Imam shaafi, Imam Ahmed (ra) said if
travel is less than 48 miles or less than 24 hours then Qasar is
not allowed but Ans (rt) said Qasar is right if travel is more than 15
miles. According to Umer (rt), Imam Uza'ee, Imam Zuhri (ra) Qasar
is allowed if your travel time is one day but Hassan Basri, Imam
yousaf (ra) said it should be more than two days. And ibn-a-Umer,
Ibn-a-Masood, Usman (rt), Abu Hanifa (ra) narrated that travel time
should be three days.

According to Imam Ahmed if you are in travel and want to stay
somewhere for four days then you have to offered complete prayer,
but Imam Malik and Imam Shaafi said it is allowed if your stay is
more than four days. And Imam Uza'ee said duration should be 13
days, Abu Hanifa said it should be more than 15 days.

Imam Abu Hanifa said when you are in battlefield you should postpone
your prayer but Imam Malik and Imam Soori said should be offer with
indications. And Imam Yousaf said fear salah was only for Muhammad
(saw) era.

Here is only one agreed condition that if you are compelled to stay any where then you are in travel condition up till you leave that place, so you will offer Qasar Salah, may its duration would be one year. On the base of this principle Imam Ahmed allowed war prisoners to offer Qaser Salah during their whole prison-period.

The above quoting are enough to know that Suhaaba Ikram and Muhaddaseen were not agree with each other and had contrary views about this matter. Every one has one's personal opinions. It's obvious that we could not reach to the conclusion that what is right or what is wrong through these quoting.

How we can get the reality? There is the only way; and that is divine order. Let go to the words of Almighty; the real and certain guidance.

The most surprising thing is when Quran is presenting very clear picture even then ancient have different opinion. Why they preferred the words of persons instead of Almighty? Why they did not considered the verses of Allah? Not understandable. As I think, there can be one reason, as Imam Shao'ba said "as much you closer to Rivayaat, would be getting away from Quran".

Quran said

Guard the prayers and the Mid-prayer (or the prayer on its
perfect timing - Salat-a-Wusta), and stand obedient to God.
(238/2)
And if you have fear, then (pray) afoot or mounted; and when
you are secure, remember God as He has taught you what
you knew not. (239/2)

The amendment in the usual way to offer your Salaats is possible but only when you are in the condition of fear but when you are secure then wherever you are you have to pray as the normal way.

These verses are explained in detail in Surah Nisa

And when you "go forth in the land", it is no fault in you that you
"shorten the Prayer" IF you "fear" that the unbelievers will put
you to trouble; the unbelievers are surely an open enemy to you.
(101/4)

Means you could shorten your prayer when you are in travel and you have the fear of your life. Both the condition should be existed same time.

Next verse is explaining the way to offer the Qasar prayers in such conditions.

And when you (Muhammad - saw) are amongst them, and
conduct the Prayer for them, let a group of them stand with
you, and let them take their weapons; then, when they have
prostrated themselves, let them be behind you, and let the
other group, who have not prayed, come and pray with you,
and let them take their precautions and their weapons. The
unbelievers wish that you should be heedless of your weapons
and your baggage, and then they would wheal on you all at
once. But there is no fault in you if you are hurt with rain, or you
are sick, that you lay aside your weapons, but take your
precautions; God has surely prepared a humiliating punishment
for the unbelievers. (102/4)

The above verses are enough to know that the "base" of Qasar Salah is the fear of enemy. The whole discussion is about for that specific time when you are out of your homeland and you have the fear of your life, then you Qasar your prayer as Allah told you.

And the orders in next verse are making the matter crystal clear in all aspects.

Then, when you have performed the Prayer, remember God,
standing, sitting and on your side; and when you are SECURE,
establish the Prayer in NORMAL WAY; surely the Prayer is a timed
prescription for the Believers. (103/4)

I think, now, the matter is clear that to shorten the prayer linked specifically with "Fear" + "travel". If you are not facing both the things same time then there is no way to Qasar your prayers. In peace days wherever you are you have to offer the prayer in normal ways. Don't you be secure when you travel in peace days? And when you are in state of war offer your prayers as you could; afoot or mounted, and if it is, even, not impossible for you then law of compulsion would be applied, that you are stopped to offer the prayer so offer it when you become easy.

We have sent down to you the Book with the truth, that you may
judge between men by that God has shown you; but be not an
advocate for the traitors; (105/4)