I'm also still in some IRC channels from way back, and I can't deny I hate what discord did to the IRC community. ("Look how they massacred my boy...")

Of course, Discord pulled it off for a good reason - the experience was seamless, featureful, rich, and modern - none of which IRC can claim. And it's only cantankerous sticks in the mud such as I who care about ideological concerns like interoperability and open standards.

And another thing that Discord did is to absolutely explode the channel count. In the IRC days, a particular community or friend group would make do with one single channel. But that group moves to Discord, and suddenly creates a general channel, announcements channel, music channel, games channel, cooking channel - all for one single friend group, and multiply that by the number of groups you are in - because the Discord model permitted it and made it frictionless.

And I think that's why for some people who use Discord at the moment, it wouldn't be enough to simply have a channel on a public Matrix instance. People are used to having a whole 'server' to themselves (of course discord 'servers' aren't servers, but let's set that aside) and so they'd need at least a 'space' in Matrix, being the more reasonably named analogue.