I agree with you in principle to an extent, but many would argue that standardized cookie-cutter education produced what gains we had and education policies which tried to accommodate diversity of thought are responsible for the declines due to lack of rigor and any real standards. "Reading by vibes" [1] directly led to declines in literacy, and there are attempts in math to accommodate different styles of thinking which may be undermining rigor there as well.

I suspect teachers are caught in a catch-22 of only being able to teach what's on the curriculum and nothing else, so they have no room for flexibility to engage students, while also being under pressure to produce high grades; but when the curriculum is garbage and the students aren't interested...grading standards tend to be what's compromised. The schools and parents are happy their kid has A's, teachers get to keep their jobs, and students may graduate highschool without having ever read a book cover to cover. [2]

1: https://www.newyorker.com/news/annals-of-education/the-rise-...

2: https://www.theatlantic.com/magazine/archive/2024/11/the-eli...