I agree with your disagreement. While I'm sure Zappa enjoyed the savings he may have encountered by holding auditions and bringing in new, young players every few years, it was more about getting more hunger and less attitude from his musicians that made him go through the process all the time. Established players have a style, and a bit of an attitude about their style, good or bad. Established band leaders want a player to come in and play what THEY had in mind and not always how the hired player interprets it.
Yeah, Zappa would probably have used any savings to finance his classical work

In Zappas case it probably took a lot of stamina and absolute dedication from his musicians to stay for any length of time, hence another reason for a lot of changing personnel.
I read an interview with George Duke, who told a tale about a heavy jazz solo that he had to do during his early work with Zappa. Apparently Zappa told him "That's great George, but could you try to get a bit more humour into it"
Jazz is not dead, it just .........
Bruce Lindfield said:
I wouldn't put it past Miles - he was a wily character and even when they were making "Kind of Blue" they didn't get much money!!
I think in Miles' case, it was more a question of his regular band members falling by the wayside due to drink/drugs or leaving to lead their own bands; so he had to constantly find new players, who would do what he wanted and go along with his "vision".
So - Coltrane, for example, had his own vision and a definite direction that was different from Miles - then later, Wayne Shorter and Joe Zawinul left to form their own band - so Miles was constantly in the position of starting again...?
I guess to some extent money would be a consideration, especially in those days, but I still think that this would not be the main reason for using younger, fresher players, I wouldn't imagine that Chick Corea is short of a few squid.
I like your last point, I think this was also true in Zappa's case, they would use the best young, "undiscovered" talent as they would go along with the vision. However being very capable & strong willed characters themselves they would probably see the opportunity to launch their own solo careers on the back of it. Of course working for someone like Miles or Zappa would be a very useful learning experience in how to go about doing this and would give them the kudos to attract the initial sponsorship to do their own thing, Tal being the original example in this thread.
So it works both ways. I still think that music is the real winner here, things can easily get too staid & comfortable, young blood brings in new ideas, approaches, enthusiasm etc, if it's guided by experience then all the better.
The next generation always goes that one step further, their starting point is the previous generations one step further.