I make no assumptions. I lived through that twice. When Mellon Bank sold out to Bank of New York (2008-ish) and when the IT company I worked for was sold to a larger MSP (2015).
Mellon told us after the deal was done, so not quite exactly telling us first. But, the exact scenario I described did happen. I opted for the short term and took another job right away.
The IT company owner told us he was thinking about it. He made the deal, the new company started blowing people out 4-5 months after the deal closed.
It just happened to me a third time this past November, but that owner offered no one any severance packages. He just assured us no one would lose their jobs.
He laid the deal out for our team (23 employees not including him), minus the $$$ details, and asked us all how we felt. Two people quit within the month. Everyone else is still here.
It's been my experience in these situations that the buyer has the technology/skills/staff/resources to run the company they bought, so it wasn't completely necessary that every single legacy employee stick around to keep the company they just bought from going under.
-Mike