This is a good analysis, but you're missing another angle: quite a few kids are not developmentally ready[0] for algebra in 8th grade but most are ready in 9th grade. By forcing kids who are not developmentally ready into algebra you generally do more harm then good (most of those kids will hate math for the rest of their lives). This problem also occurs earlier in school life with fractions. When I was in education I saw pushing students too early like this do a lot of damage that had to be cleaned up by tutors at a later date and was totally unnecessary.

I even think Calculus could still be achieved by the end of high school with the right curriculum choices - you can teach a lot of calculus concepts to grace schoolers if you elide the actual mechanics of computation. Anecdotally, students who were pushed through that sort of curriculum earlier in middle school tended to breeze through high school calc.

[0] https://en.wikipedia.org/wiki/Piaget%27s_theory_of_cognitive...