> - use your preferred client to connect to your preferred server.

I want to like Matrix, but this is currently the most painful point, for me. There are very few mature clients, so they don't serve as many niches or specific tastes as IRC clients.

> I can't see how IRCv3, if it ever lands, can compete with this. It's years (decades?) behind at this point.

Not sure what you mean by "landing". Many specs are already implemented, deployed, and used in the wild.

> And if it lands a spec which is equally capable as Matrix, how can it ever be compatible with "the old IRC"?

Every IRCv3 spec is designed with backward compatibility in mind, so old clients are not left behind, they just don't benefit from the new features. The main mechanism for this is a capability negotiation when clients connect.