Im not from America so our economy is nowhere near yours horses are selling decently enough here and I am not looking at breeding for anohter season maybe 2 depending on stallion quality.
We have bred before I stated that so we have all the night cameras extra extra large breeding sheds and 24 hour vets on standby so not sure where you are getting your information from. Its HER first time breeding, I like a longer back in a maiden after dealing with short backed ponies and horse I find they carry easier when they are longer.
To my understanding, it is fairly common knowledge that a shorter coupled horse with a shorter back has more strength and mobility throughout their back. They are better able to reach under themselves with their haunches and round out.
I don't feel that a long back can ever really be considered a bonus in a horse and I don't think it can be framed that way. None of this is meant personally of course. My warmblood mare has a long back and weedy haunches sort of like yours, so I deal with the same sorts of things... though she isn't the one that I would ever choose to breed if given the chance.