I don’t think it’s necessarily about ease of use, it’s more about ease of setting a thing up, and getting people to adopt the new place.
For discord, there’s one platform and anyone can start a ‘server’ and then create ‘channels’ and issue invitations. And as far as I know, it’s free.
It’s true that Discourse now has a much easier to operate setup for custom domains… but the initial admin still has to choose and pay a hosting company, or pay someone for a hosted solution.
And the admin needs to understand local law, and the users need to trust the admin to keep the place going.
Migrating communities is difficult, and (I would say) never 100% successful - some people will migrate, others won’t.
Another question might be, where will Discord communities go… whatapp groups? signal groups? threads? bluesky? Many choices.