I can support this from the other side. Coming out of a maths degree at a reasonably good university, a lot of the mathematicians who became teachers were those who struggled and pretty much gave up at some point in undergrad. Pushing many of those who will become teachers to mathematical breaking point seems like a bad way of doing things, and I think contributes to this. It's hard to think of a good alternative that still produces enough teachers though.
Only thing I can think of is to pay maths teachers really well, which probably means increasing class-sizes, so is a no-go at least in the current political clime of the US.
