In practice "better UI" would mean things like being able to trivially share files and images, or quote/link a specific message, or even making it easier to distinguish between users with similar nicks via their profile pictures. And those UI improvements are actually features which are integral to its protocol, so they can't easily be bolted on by a custom IRC client in a backwards-compatible way.

Literally every single modern chat platform has support for stuff like that, and for a reason. Discord became popular because it combined those modern chat features with the ability for every community to create its own private little "server" - while at the same time making it trivial to participate in multiple "servers" at once.

I'd guess the important feature for Discord is it is easier for the administrators to get hosted and online, but "you could create a client with a better UI than discord" is a terrible line of argument. People could do lots of things in the OSS world and they don't. I can't recall any IRC client that I have found as easy to use as the Discord client except - ironically given the topic - ChatZilla which died off years ago because Mozilla decided that extensions were more of a 2000s technology than something they wanted to support.

Email is OK. The point is that most conversations moved to other media (mainly chats) and so 90% of my mail is notifications, 9% is newsletters, 1% are real messages. They used to be 99%.

indeed, there is https://www.irccloud.com which is quite excellent!