Earlier quoted context omitted.
"Mainstreaming", which dates became a buzzword around 1980, refers to putting special needs in the same classes as other students. It does not generally mean that everyone was on the same track. See also https://en.wikipedia.org/wiki/Mainstreaming_%28education%29 For example, this 1995 article describes how a school for the blind was closing because of the mainstreaming of blind students. https://news.google.com/news…
The number one reason is that they can fire bad teachers and require quite a lot from those they hire because they are not in the union. You are assuming it's about the curriculum it's not. It's about the environment, the parent participation etc.
Therefore I can't see why you think I assume it's only about the curriculum.
Nor do I see how unions are relevant. Your 1960's classroom, in the cartoon that resonates so strongly with you, was almost certainly run by a union teacher, in a time when unions were much more powerful than now.
You can't decide to choose one part of history and not the other.