I'm still not entirely sure on the point of IRCv3, it seems to me that the main draw of IRC in current times is the simplicity of the protocol, losing that by adding a whole bunch of new features, I don't see what niche IRC fills anymore.
The second is more green field projects, like the stuff to allow a more web client friendly protocol compared to IRC currently by defining how to use web sockets avoiding the need for stuff like IRCCloud or Mibbit proxying their user's connections. Or chathistory to make it more mobile/not permanently attached friendly.
It's niche is as simple as it was way back when, you can create your own chat server for your community. Instead of having to use Discord, Slack, etc you can create your own and control it fully. While it's possible to do that to this day it's not to the same level it was back in the late 90s for example.
IOW we're stuck with a decaying protocol ecosystem—for messaging, and everything else—until we outlaw surveillance capitalism, which we probably won't do. Email wouldn't be able to take off if it were invented today, because it lets you use it without one company seeing everything. The state of things is really bad. It's a drag on productivity, with endless wheel-reinventing and deliberately-bad interoperability.
The success of slack and the failure of IRC has nothing to do with surveillance capitalism.
Minimalist implementations can also cherry-pick newer features they want. For example, this IRC client explicitly says it in its "non-features" section: https://git.causal.agency/catgirl/about/