This is an interesting question. Wikipedia claims 230,000 users at peak times which is still quite a bit more than say, gopher. A 98% drop is real but you'll still see IRC occasionally for software purposes (like say, Debian)

Maybe it was a coalition of people there for different purposes and some of those groups have fallen away for different places.

For instance, people were doing dating and sextalk on irc back in the day along with file-sharing. Those applications have been superseded by many other places. I don't expect to see anyone sincerely asking "a/s/l?" in modern IRC chatrooms.