Best case for their charity is likely to stay on Slack to get them as corporate sponsors, but add "data sovereignty" as a core part of their curriculum. That keeps the charity going, but instills the foss ethos in all their students.

Who knows though. They have 5 years of free slack either way because it would be incredibly bad PR to revoke it now. The runway is there to exit cleanly at least, which is a lot more than they had before.