I'm not sure that failure is necessarily the form in which such a problem would manifest itself - I've been around enough businesses to know that they can muddle through personnel changes. But it tends to impede short term progress and create stress for those remaining after a key employee leaves - the sorts of problems the alleged no-poach agreements allegedly sought to mitigate.

Now there may be something else that motivated Altman's essay, but it is almost certainly related to YC and the idea of improving employee equity appears to have been seen as a potential solution to some trend in their data. I think that YC came to believe turnover was impacting rates of return, and YC has had to think about ways to mitigate it. They're at the point now where they would have that data and some companies in their portfolio are mature enough where brain drain has a significant impact. For example, they could be running regressions around on former employees of portfolio companies applying to YC against the return rates of investments in the former employers.

There's the pursuit of better returns in there somewhere, and I reserve the right to pull another theory out of my ass at a later date.