It does, however, create a pipelining problem: there's very little reason for someone who wants to become a professor to stick through literal decades of adjunc

It does, however, create a pipelining problem: there's very little reason for someone who wants to become a professor to stick through literal decades of adjunct hell to maybe get a tenure-track position when somebody decides they're finally ready to let go of theirs (and the paycheck that comes with it, which is not large but certainly incentivizes staying a long time). Tenure is a great and necessary phenomenon, but it is one of the best examples of the folks who get it needing to be more broad-minded than themselves.