(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.)
that's a perfect teaching occasion, then!
Kids: don't use proprietary services just because they are trendy. Prefer always open standards!
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.