No, discord is not a good alternative. We just have to wait until they pull the rug from under their free users in order to aggressively monetize. And I think that will hurt much more than Slack.
not just that, they're also incredibly hostile to users trying to retain privacy.
they're requiring phone number validation just after registration or you run the risk that they suddenly lock your account, demanding a phone number for "abuse prevention". depending on ip reputation and possibly other factors it may be impossible to use it in the first place without providing a phone number.
I like, and use, Discord daily, and like it a lot.
But I have to agree: Eventually, management will start to more aggressively monetize it (i.e., "pull the rug from under their free users").
Truly free (e.g., IRC) alternatives seem like the only long-term solution, otherwise people will keep stumbling from one commercial platform to another, because they never last.