It didn't seem that obvious a term as you described it here, but a bit of googling gave the background:

It's effectively that the more desirable elements leave, the less desirable parts stay behind and get more concentrated... So the bad employees are the salt in the dead sea

It was coined by this chap https://en.wikipedia.org/wiki/Bruce_Webster#Career

MS did a round of layoffs that was specifically targeting low performance last year: https://www.cnbc.com/2025/01/08/microsoft-confirms-performan... and there's no reason to believe they wouldn't do it again. I doubt that anyone there is unaware that hanging on isn't a realistic option if they're not delivering.

You get the same with government jobs. Anyone smart leaves and gets a better job, so you’re left with the dregs.

I termed it "Sh*t floats" when I was in the Navy.