Not irreplaceable. There are many other very good, very qualified machine learning researchers, and Ilya relies on the people under him just as much.

Is he, though? Genuine question - I am sure the Chief Scientist (or equivalent) of Xerox Parc was a brilliant person, but if we take our engineer blinders off, a successful birthing of products that millions benefit from is from more than just the genius of the tech/product person.

Ilya is done. This is a clear instance of the chef who made the secret sauce breaking down after the restaurant expanded. Except now, regardless of his contributions, there's enough people making similar sauce and not poisoning it that it'd be better if he went back to some idealistic mom&pop so that the business could bring in line cooks.