Earlier quoted context omitted.
The integrations are a neat trick, but often end up spamming a channel where people are trying to communicate. Teams seamlessly works across platforms, and mobile with video chat, files, outlook integration, and screen sharing. Maybe IRC could of been that, but it never happened.
There are nice IRC clients out there. I don't use them and I haven't used them in a group, because even in programming language communities many have moved to Discord/Slack, but I would prefer working with multiple simple tools than a monolith, different tools for different uses, and glue them together when needed (it doesn't need to be complicated, and if it is, than some additional effort is justified)
The rest of the shift is explained by discord being popular for other use cases (e.g. as a twitch companion), overwhelmingly dominated by younger users. IRC isn't getting new users and over time, this means the community will migrate due to turnover. Thank god we at least have things like matrix bridges, but discord is a lost cause.