As I understand it, the IntelliJ version you can keep using forever is the one from the beginning of the subscription.
So while the subscription was valid, you got upgrades, and then if you don't renew, you have to downgrade. It's their way of nudging customers to keep paying. It's working for me.
This is a bit misleading; you only get a perpetual license for the version you had at the beginning of the 12 months, as you can see with the second graphic[0] on that page.
The version is refreshed on an annual basis of payments. So your fallback version will always lag, but it won't be significantly out of date if you have a 4 year old account, for instance.
So while the subscription was valid, you got upgrades, and then if you don't renew, you have to downgrade. It's their way of nudging customers to keep paying. It's working for me.