I liked the old "pay for updates" model. Nowadays, that seems less viable because security updates are so important[0]. I think the middle ground model would be something like security updates are free, feature updates cost money. Something like pay $x for version n+1 or a discounted rate for a subscription.

Sounds like a bit of a hassle on the logistics/release engineering side, though. That would need to be handled with some care and planning.

[0] Which I'll admit I don't 100% buy, but I'd love for a security expert to weigh in.