Already happening in the humanities. Many major programs in my field when I was applying (as an international student, so there already were some "quotas" in place on how many international students a program would take) were signaling that they were reducing the recruitment number simply because they could no longer insure TT placement afterwards. It's too little too late tbh, but at least they are starting to do it. I know they are starting to do it in non-humanities programs as well.

I'll add as well: the standards expected for present day PhD students is insane compared to PhD students a generation ago. When I speak to some of the tenured professors I know, they readily admit that they would not meet the standards for TT positions today. Add to that the insane number of applicants, and the tendency for hiring committees to default to applicants from top-tier programs for hiring (regardless of the quality of their research), and you end up with an increasingly bubbled and desperate profession that is extremely unwilling to break from party line and disrupt whatever sliver of a living they ended up in.