I actually don't think IRC is doing that well. I used to have a bouncer but I shut it down a few months ago because most of my communities had moved over to Discord. But I think the reason why Discord and Slack took over was not because IRC was decentralized, but because the protocol was never designed for mobile devices where you might not always be connected to a server.

It didn't help that the protocol evolved at an absolutely glacial pace with very uneven support across various networks, and doing anything but the most trivial sorts of channel management had to be done through a bot. Matrix seems like the obvious IRC successor due to its seamless two-way interoperability with IRC networks, but I suspect that it won't hit critical mass until Discord starts causing problems for its userbase.