> I'd guess the IDE have now reached this "fully featured" milestone where most developers don't care to upgrade. So IntelliJ has to switch to a subscription model to survive.

Sorry but this doesn't make any sense to me and I doubt IntelliJ's sales are putting the company in any danger.

Even if they couldn't come up with new features, the programming languages and frameworks keep evolving and the tooling needs to catch up with that. But I can come up with a hundred things to improve from the top of my head, so I'm sure they can too.