IRC offered a unique featureset for many years - nothing else offered the same global scale and large-group-oriented chats. People will put up with very poor usability if it's the only product with a feature they need.

Yes, large distributed groups where people can drop in/drop out is IRC's niche, and why it will never die. I prefer IRC for large open-source projects to Slack, but the usability is awful for a company environment. At my school, we use Slack for research groups, study groups, and student project teams, and it's excellent for that.

I'm not convinced IRC will never die; for a while it was the only thing that covered that use case, but these days there are other options (e.g. Discord).