That's precisely why low achieving students are separated out. To give them extra help.
High achieving students are easy. Just point them and they go. This is why the second they started standardized testing and separating the students they were able to achieve results with high achievers. But their primary goal with these top-down programs was to actually help the slower students, that were graduating without being literature and whatever. Turns out it's just a really hard problem. It isn't that everyone in education somehow lacks the desire or common sense.
> Nobody is talking about forcing the high achievers to do the teacher's job.
On the contrary, people can be quite explicit about this.
> The other option is to help the low achieving students so that more of them can participate in that winner take all economy. I'm not sure how to argue that this latter option is preferable since it seems so obvious to me that it's the right choice to make.
Why do you think society is becoming more "winner take all"? Even if there's truth to society trending in that direction, I'm skeptical it can be solved in the education system if the causes don't lie in the education system. It's easy for me to imagine a world where schools eliminate their advanced programs, then the same students go on to become low-wage workers and the same (or fewer) go on to become scientists.
Nobody is talking about forcing the high achievers to do the teacher's job. The question is how we allocate the fixed amount of educational resources we have. You want us to choose the high achievers so that these early winners can turn that lead into even greater success later. Unstated in your post is what happens to those low achieving students. But it's pretty easy to assume that they're going to be the losers in the winner take all economy.
The other option is to help the low achieving students so that more of them can participate in that winner take all economy. I'm not sure how to argue that this latter option is preferable since it seems so obvious to me that it's the right choice to make.