Working composer here. That was a fantastic idea to create this topic!
From my experience past experience working with devs they seem to prefer / need both short and long loops since they're more flexible. It seems that more complex songs (that will tell a more specific emotional journey) are usually where our custom services will better solve the problem for the devs.
However, let me tell you a different experience regarding non-loopable tracks.
I've just composed a boss theme on my YouTube channel which is not really a loop (I think it's a "hybrid loop") and it's very chaotic, with no leitmotif and a lot of melodic, harmonic and rhythmic variation throughout the song, and... it's being one of my most visualized and liked song so far - which disproves a lot of traditional ideas as to how a boss theme song "should be", since some devs seem to be looking exactly for that.
So, I think that it's important for us composers to ask our fellow devs what they want but we should also offer our own new musical discoveries to them because that would exercise and develop our creative powers in our field and also offer new / novel musical perspectives that could potentially make the devs's games stand out even more, giving them an extra edge in the fierce competition of game production.
Video game music has evolved and became a lot complex on the technological front with dynamic, non-linear music being implemented through middleware, but music is still evolving in itself in the old fashioned way - through different ways of creating the same ol' loops.
That's being done through new sound design thought processes, intellectual interpretations of melody, harmony, rhythm, space/time interpretation and understanding in the context of sound, and also through the interchange of instrumental technique studies (expression, articulation, dynamics, etc), and their effects on music composition.
We as a composers can work on both fronts and offer music products and services for devs that don't want to meddle with too much technological complexities regarding music (samey ol' loops), but with something different in said loops, and also the opposite offering them products with music stems and complex separated instrumental layering (although they seem to be a minority in my experience).
So, my question for the devs would be:
What would be the function of the required loops? In simpler words, what do you need the loops for?
Knowing what they need we could offer potential new solutions. For instance, instead of stacking 3 different loops so that a prolonged level is not boring and irritating, why not offer a music pack with "hybrid loops" of longer durations, functioning both as more nuanced emotional storytelling but also behaving in a way that's seamless? Maybe that will sound like a crappy and useless idea, but maybe some devs will find that intriguing and potentially useful.
My 2 cents.
When I have tracks that don't loop, I loop them myself. That can take hours of fiddling with the track in music player software to get the sound to continue so the player doesn't notice.
My levels can be anything from 2.5 minutes to 15 minutes long, depending on the game. One track is unlikely to cover the entire level, so I need the track to loop.
If a track stops and there's a pause before the next one starts, it breaks the player's concentration. They suddenly become aware of the change, and it could hamper the game play - especially if they're playing against a clock.
If I use multiple tracks on the same level, I need to blend them. But here the problem is to find tracks that are similar enough to sound like a continuation of the same song, but not identical, so there's variation. If there's an unexpected change in tone or speed, it could also interfere with the game play.
I can understand that a boss fight level could use its own track that is odd. But I write casual games, I don't have boss fights in my game.
Hey man, I have composed for a casual game before for a client, you can search for it on Steam as "Goosey Guess", it's a guessing simulator. Our answer to the problem of keeping the player engaged through music was to compose long tracks with lots of variations and repetitions as well, it seemed to work, there were twitch streamers saying it was good and relaxing.
So, I think that's about experimentation, my example of the boss theme there is indeed an oddballs case, probably likeminded Dark Fantasy RPG or JRPG devs might be using it, but it's definitely not a standard approach.
But it's like what you said, it's difficult to find loops out there from different composers that will combine nicely (specially if you're looking through free assets it seems), hence my suggestion for composers to experiment creating different types of music packs!