The #FreeSoftware community managed to keep #IRC relevant for decades with the same strategy - it was the only medium to chat with them. There's no reason the same can't be done for #XMPP, a vastly superior protocol.
Oh, I should mention that XMPP supports anonymous authentication, so people can chat without making an account, just like IRC. And you can use Movim, Converse.js, or xmpp-web to provide a web client. Onboarding new users is trivial.
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