in the last 20 years I have had more attacks on my email account and $whatever_current_social_network account then I had on my irc account. Things like nickserv and ip cloaks (which have been part of nearly all networks I used to connect to and those I connect to) do the job just fine. Is it easy to flood down a server, sure is, but if there's more then 1 hub on the network, things will settle pretty quickly normally.

In my opinion, the main reason why it was never adopted as a mainstream platform, is because it was never picked up by a big corp that saw a way to earn money off of it.