> Must be a pretty nice sum for them to agree to it

Not necessarily: if the investors don't agree to a reasonable amount, the wanna-be acquirer will simply hire the entire team with generous sign-on bonuses, and the investors will be left with a shell of a company.

In this case, the core product is MIT-licensed, the team can quit on a Friday and pick up exactly where they left off under a new org on Monday.

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