Earlier quoted context omitted.
IRC is terrible for work collaboration. I say this as a long time IRC user. Conversations are not searchable, and if you're offline you are unable to follow along with what's going on with your colleagues. To make it anywhere near usable, you need to run IRC bouncers, chat loggers, search indexers .. at that point, just use something made for collaboration.
Not to mention: 1) IRC doesn't support any form of message threading. There's some user conventions like "username:" or "@username" to direct a message to a specific person, but there's no way to make a clear distinction between multiple conversations in a channel. (You can make a new channel for the conversation, I guess, but that's even more hostile than usual to other users trying to follow along.) 2) IRC doesn't…
The other 3 are UX matters, 2nd point cracks me up because hexchat could split a message in batches of privmsg's eleven years ago, spam control is totally local (so being banned for spamming isn't a given), servers can nowadays buffer a lot of text and throttle messages, albeit bandwidth is mostly free.
Being them UX matters, they aren't worth much, thus IRC will remain broken because there are like two thousands IM competing software that dealt marvellously with those problems.