But maybe what should be changed is the expectation. If the vast bulk of 8th graders aren't ready to handle algebra, don't create an expectation that "the smart kids" are taking algebra in 8th grade, because parents are going to be pushing their kids to be one of the smart kids (as measured by when they take algebra), and you're going to get a bunch of kids who aren't ready.
But this brings up a problem. Who decides if a kid is ready? The parents? The teachers? The school bureaucracy? On what basis do they decide?
Teachers seems like the best bet (they observe kids in the classroom all day), but they also have biases, blind spots, laziness, and so on. I don't know that it's possible to build a system where it happens "as it should" all the time.
Neither should it be accepted a priori. It would be a compelling reason to delay math instruction, if supported by evidence - but is there any evidence to support it?
And if improving education by delaying instruction until students are mature enough to handle the material is truly the goal, why delay instruction for students who are already mature enough to handle it sooner? Holding back students who are ready doesn't improve education for anyone.