If IntelliJ doesn't get money because developers think "old version is good enough for me", then there is no money for bug fixing and for keeping the product alive. It would end up as abandonware like most apps on the iPhone App Store.
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.
So we users have the choice between paying a subscription or having the IDE end up as unmaintained software due to lack of funds.
IntelliJ can't put out a new paid "version 15" which has bug-fixes only. People would be unhappy about that too.
I feel that the core IDE has degraded in quality over the past years because the releases were feature-driven, and I'd be happy to see IntelliJ refocus on quality instead of quantity.