But you are helping underperforming students by placing them into top schools, comingled with better students - lots of research shows that. So lowering the bar for admission really does help.
I think the open question is how that balances against the cost of having fewer slots available at those top schools for the very best students.
But I am pretty sure that if, say, a student can't pass a multiplication test, but you give that student an A+ anyways, and then let that student into a "top calculus class", that's going to hurt the student more than it helps.
Ya sure the student gets to mingle with better students. But what you would usually get is, the student would end up being someone who mentions a lot of buzz words like "fractal integrals" just to fit in, but have zero idea what is going on, or what is being taught in class.