I think the one big distinction between IRC and email which made the latter gain massive popularity is that theoretically (i.e. spam-prevention and other blocking notwithstanding), any email you send from one address' provider/domain can reach any other email address. This means people can publish an email address and anyone else is able to easily contact them. The server that gets mailed is even specified in the DNS system - MX records. With IRC, you basically have to be on the same network to chat, and there's no way to publish an "IRC server address" in the DNS.

I have a feeling that if DNS had this capability, and "IRC addresses" became standardised in the same way as emails (e.g. nick!irc.example.com), allowing anyone to use it with the same universality as email, the story might be very different today.