Your perpetual license only works on a historical version. It doesn't work with any updates. The incentive is for them to continue building products that make you want to upgrade. The perpetual license key works in a disconnected network so it doesn't matter if JetBrains went out of business. You just need to make sure you have a local copy backed up.
[1] It would probably be difficult contractually, but such things happen. See e.g. TextDrive hosting lifetime accounts.