I agree to a point, but… The WB conformation is not ideal for many other disciplines. And even in that inspection world, there are traits that are desirable because of the performance standards of jumping and dressage, that are not desirable for (a) many other disciplines, and (b) long term soundness. Not really all pointed at your post but…
As many have pointed out, uphill does not create speed, so that would be one trait that is not desirable for the QH or TB who are both speed horses. That is just one example.
And in regards to long term soundness - big movement, and overly big horses do not lead to long term soundness. And big movement is NOT desirable for many pleasure disciplines, it is too hard to ride.
There are many other examples of both (a) and (b) as well in the WB inspection world. OTOH, it is a better model then many other registries use, because there is some level of quality control (beyond the show world).
You can’t use the standards of the WB (sport horse) conformation and say it is ideal for ALL breeds and disciplines. OP, you probably need to develop some understanding of other breeds and disciplines to do the study you are discussing. It would actually be very interesting to get some perspective on the whys of different conformation characteristics. And to see the situations where those characteristics have been taken too far - and situations where we’ve lost sight of less obvious characteristics (such as temperament).
Another interesting conformation characteristic that you might look at and say WHY? Andalusian horses tend to have very narrow, upright feet - very SMALL feet. I have always wondered why that was something that was bred into the breed. Then I read an indepth history of the breed, and they pointed out that this is a breed that originated in a place with dry, hard, often rocky ground. So the horse’s feet grew tall and deep to provide protection for the foot and frog. Bigger feet would just chip and fall apart, and wouldn’t handle the rocky ground well.
Or, when you look at many draft breeds, you might ask - why such a sloping croup, why such a short, thick neck? But they are bred for power to pull (versus push), and those characteristics allow them to pull heavy loads.
Gaited horses are built for SMOOTH movement, so a person could ride all day long in comfort - different then big WB movement. QHs are also bred for short bursts of speed, then comfortable all day movement. Arabians are bred for long distance speed and hardiness in harsh climate. And so on - different breeds, different purpose, different conformation.
You might even ask, why do some breeds deviate so far from the wild horse, which is nature’s answer to sturdy, thrifty, hardy?
Having said that, the other thing I would point out is that halter breeding (in any breed, and for that matter, in any species - look at what we’ve done to DOGS) seems to deviate a long ways from functionality. Halter breeders and conformation breeders (horse, dog, cat, whatever species) do tend to take characteristics too far, and create non-functional animals.