This is true.
However IRC complements forums in my experience. Certain communications are more suited to forums, others are more suited to IRC.
For example: for someone unfamiliar with bash, trying to get a shell script working may require a dozen questions on IRC, together with pastebinning console output, maybe screenshots, ... This would be impractical on a forum.
On the other hand a question such as "how to build libpython on android" may be unsuitable for IRC, as it is a long detailed process that doesn't require this bidirectional to and fro.
Sometimes I ask a question on Stack Overflow, paste the URL into IRC, have a conversation on IRC, and then answer my own question with the knowledge I have gleaned.
Additionally, one can log IRC channels. I am logging #juce.
It seems no one has yet designed a truly modern information handling system, something that merges the 'real-time' of IRC, the 'community votes things to the top' of S.O. and the 'threads' of forums. I'm going to have a crack at that one day if it ever makes it to #1 on my priority list.