Multimedia, end-to-end encryption, offline chat history, and having all those features actually available and usable by the average user (just having a persistent identity is something IRC makes trickier than it needs to). You may decide those are misfeatures, but most people don't.

Meh. IRC now can use TLS/SSL (Freenode), among SASL.

On X11 vulns, I use a tty, so I don't care, and even on X, xterm has a secure mode.

Us cabal devs really like irc. Cabal was inspired in part by it. You might enjoy this explanatory zine:

https://substack.net/zine/cabal.html

Different goals. IRC needs a central server. This is peer-to-peer.

IRC servers are peer to peer, so it's a kind of archipelago architecture. But they trust each other so it has to be coordinated and built on trusted relationships.

Does IRC have "conversation threads?" To me a conversation thread necessarily entails a hierarchical chain of posts and the replies to those posts as "child nodes". All conversations are inherently "tree structures" if done right, imo.

some people want more than plain text

Not user friendly. Not everyone is an internet nerd like us.

That's tentative I feel. Anyone can go to https://webchat.freenode.net/ and join a channel someone posted the name of, "join ##channel_name_here on Freenode". Registering with the nickserv and what not? For some I'd agree but it's not entirely obscure.