(They do help clubs sell things, taking "7% of income", so they do have a revenue stream, but the money that Slack wants would pay a veritable army of student interns.)
So if you use an open standard, but not self hosted, and your provider tells you "pay 250k or lose all your data in 2 days", I'd say are not necessarily in a better position than they are now.
It's not impossible to migrate off of slack, but migrations take time.
I know this, because I've done it.
Similarly a migration from self-hosted to SaaS gitlab (though, not back).
Perfect is the enemy of good, but man, it can be pretty close to perfect if you choose your vendors properly.