It's a relief to finally read that Discord is indirectly shutting down and getting rid of it's users. It was inevitable but dragged out far too long with all the VC money to burn. Hopefully everyone can figure out how to use XMPP and/or get back on IRC. It is a genuine shame how much culture and information will be lost inside their walled garden though.
XMPP and IRC are great and all but a massive part of what people use Discord for is group voice calls with screen-sharing. I'm not sure what the alternative is for that. TeamSpeak is the closest I can think of but it's not a 1:1 replacement for a number of reasons.
We use Jitsi for voice/video calls + screen sharing (but I realise you may be talking about all-in-one alternatives)
it's possible to integrate jitsi in such a way that the chat has a function that will open a jitsi room and share the link to that. on irc this could be a bot. for people used to irc that's seamless enough. for something more convenient you'd want to integrate that feature into the chat client interface such that it can track who is in which jitsi room, etc...
of course this has yet to be built.
IRC is a much more impoverished chat experience than Discord/Slack in a bunch of ways. Suggesting that people "get back on IRC" is not a serious proposal for making it possible for groups of people to chat online without being subject to identity verification or censorship.
I'm afraid the thing that would be replacing Discord will be even worse
I think it is more likely that the next best platform will end up gaining three principled civil libertarians and about seven zillion witches.