Not necessarily. There are likely key person clauses in the prior round docs.

It has happened in the recent past (2-4 years ago?); I can't remember both the acquirer and the hollowed-out startup, and searches are returning chaff, but it got to the HN frontpage.

IANAL, but at-will employment cuts both ways- thr best an employer can do on behalf of investors, are golden handcuffs - and people can be bought out of those.