Yes comp could help to attract better talent, that's what Facebook is doing. But of the about 150 interviews I've conducted for Amazon, 90% of almost all candidates always ask me: "So is it really as bad as they say working here? With how they treat you?"
I think that's a pretty good indicator that the reputation is just tarnished, and I wouldn't be surprised that that's having a sizable effect on the decrease of talent.
The other thing is, yes maybe some real bad apples leave from a PIP, but the whole culture around it, the stress, the feeling everyone has that they constantly have to fight for trust and respect, that is also a cause for a lot of the really good engineers and high performer to leave as well, of their own, no PIP involved, but it's the same root cause for why they leave.
I see so many good ones, ranking high every year, and after 3 to 5 years say: Well I had enough of this BS, too much hassle always playing the game. If anything, that's the biggest issue.