The "AI" service consumes real resources - it is not free for them to operate, and is a new cost burden that cannot be absorbed by the existing pricing. It makes complete sense for it to be its own cost.
You can also use the offline-only mode, which is 100% free and just runs on your machine.
I hate the "everything as a subscription" model with a passion, but JetBrains is one company that does appear to do it the "right" way where you're paying for continued development, not continued access to the same version.
> Well that's what happens when some mysterious new plugin finds itself on your dev tools without much acknoledgement
It was in the changenotes with a huge section, it was in the newsletter, and the blog.
JetBrains releases a lot of features of plugins, as they always have, so you can choose to slim down the IDE or replace functionality with 3rd party stuff (like copilot).