Professors are "supposed" to help avoid the retirement effect by not accepting new grad students, in some cases where this is not viable they can continue with an emeritus title and provide the entry point that their students need. Similarly, a professor leaving for another institution will likely make right by their students.

In the case that none of the above is possible the institution will probably try to do right by the student, but it's a bit of a crap shoot. Even if the institution confers the PhD, the student is still beholden to the publish or perish culture of academia - and their (former) advisor would have helped navigate them to a thesis which their former advisors peers would accept. If the latter doesn't occur in a big way; then the student won't have a good career ahead.

I've known 1-2 people who for various reasons decided that the grad student life was the optimal career station for themselves as it provided the benefits of an academic career without the overhead of grant applications etc.