It's not like you couldn't create an IRC-client with better UI than discord. Not as many features, but whatever strength discord has it is not UI.
Email really could have been great, but html and bad actors have made it so much worse than it needs to be.
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.
Quoting/linking is a client feature, not a server one.
IRC servers do also support profiles.
I think the real “issue” with IRC is that its users generally prefer the minimal UI. So there isn’t an high enough demand to make prettier UIs. But there are web clients that are a little less basic.
For what it’s worth, I’m in that minimal camp too. I wish I could still connect Slack to IRC.
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%.
I really wish Google's Wave went somewhere. It was the real solution.
Which feature did you like most?
try deltachat. it's essentially a chat client with all the features you would expect but using SMTP as the protocol.
indeed, there is https://www.irccloud.com which is quite excellent!