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.
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.
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.