question-
how much is it there not being enough trainers who know what to teach and how to teach it; an how much of it is the fault of the promoters, who want to get a kid out there and maximize their earnings. Or the fault of a fighter who either a)is so limited he can't develop the skills fast enough to be competitve OR worse yet said boxer has already reached a certain level of success and isn't willing to take direction, instead seeking to get rid of said trainer.
Yeah, I'd like to add: one thing to consider is that although many trainers still have old school knowledge, what matters are the trainers who raise fighters from a young age and feed them through the amateurs into the system required to sign with the big promoters and hence make it to the big stage.
Coaching boxing is a craft that is passed down from master to apprentice over the course of many years. Like spoken languages and native cultures, such crafts become endangered in the modern comformist world of "here's the popular/right way to do it, everybody do it this way." The modern world is simply ill fitted for professions involving the master/apprentice approach.
In Muay Thai, only a select few camps can send fighters to the big stadiums. So if all of a sudden those few camps change style, well everybody wants to get the big stadium fights and they have to go through those few camps, so the whole sport of Muay Thai changes.