>The real risk of drain is people leaving their fields of expertise to never return.

That is happening right now, all the time! Especially in the biomed field! Many, many PhDs spend 5-8 years getting their degree and receiving minimal pay, then 4+ years being nomadic postdocs, also making terrible money, only to eventually arrive at the end of the road and realize they have to do something completely different.

It is unsustainable for every professor to train 10 PhDs in their career, because there aren't going to be 10 professorships (or even 3) for those PhDs to fill. Funding has to grow at the same exponential rate as the number of researchers. It did, from roughly 1950s to 1980s, as the university system expanded to accommodate the Boomer generation. It has slowed since, and the PhD to professorship pipeline got longer and leakier. It's doing a tremendous disservice to the bright, well-intentioned young people who join PhD programs.