I think you're talking about two different things. In ianai's example, the goal is a tenure-track professorship. In yours, it's essentially "get a PhD and see what happens." Both are perfectly fine goals, but they are different.
Makes sense - I’d just not encourage someone to quit or not get a PhD because the odds are against them for a tenured position. At this point a tenured position is something you luck into (after a hell of a lot of work and politics), but way too much is out of one’s control at the start of a PhD to make that a goal and reason for getting the degree. It’s like letting your probability of being an F1 driver determine whether you take an advanced driving course. Maybe that makes sense, but I’d more heavily weight how much I like driving in the decision rather than the potential for being an F1 driver.
The numbers don’t work at all - one tenured professor might produce 30-50 PhDs in her career. Only one will replace the professor’s tenured position. If just 5 actually go on to run labs somewhere else, they may produce another 50 each, leading to ~250 qualified people aiming for one of six positions, which are filled until the 6 retire.
There's definitely a range between tenure-track professorship and professional sports as careers. Plus I think there may be some wider lack of knowledge about just how more competitive professorships at colleges have become recently. I seem to recall it's in the 10s to 100s of thousands. Professional sports is much more scarce. Probably those two help flush out what a range of acceptable odds to aim to overcome and maybe not looks like.
Most PhDs (all?), though, are beyond the diminishing returns to additional outlays of effort. Diminishing returns are probably a good metric to inform people about, too.
I wonder what the ratios look like, if you looked at competitive D1 athletes <> professional sports openings compared to PhD-bound grad students <> tenure track position openings.
