For a few years, he started checking the median age of physics professors, and noticed it went up every year instead of down. Tenured profs decided to stay in their seats instead of retiring, and there were no new positions being created for young professors. That was his indication to get out (this was during the mid 90s).
The real crux of the problem is that modern PhD programs were originally designed post WW2, in the era of the GI bill and fast expansion of college education where there was high demand for new professors. That's no longer the case, but we still train PhDs as if they would get a job even though most won't.
Still, it seems low IMHO. Unless you start counting at the point where the person gains full tenure rather than from the point where they become some kind of "group leader", "assistant professor", or such "tenure track" type position?
Why? I know an academic who has done exactly that. Nearing retirement now.
Does physics academia really not have another position you can promote someone to, if you want to keep paying them to do research for you, but they've "timed out" of the postdoc role? I had assumed this kind of research-scientist role wasn't CS-specific, since it's existed for a while, and at big universities, CS hasn't traditionally had the clout to invent completely new job titles, so hiring is usually mapped onto stuff that exists in some form or another at the institutional level.
Eventually you need the money for retirement...