Under-appreciated factor: the problem with decentralization is that it pushes work on to the end user, who is least equipped to deal with it. People actively want centralization of things like anti-spam because it lightens the load. The fact that this gets paid for in insidious ways rather than directly paying for a service causes all sorts of weird market distortions.

Note that Discord doesn't replace IRC, it also competes with TeamSpeak; there's a whole voice and video sub-feature to it. Not everybody uses it but the fact that it's available in the same software was advantageous to the original market, gamers.