This is an interesting topic, and I believe the solution is actually not so complex and already used. It's SaaS.
Users should always have the last version of the product. It is not possible to have users on a version that is 5 years old. Google and many other understood that so they have automatically update on Chrome for instance.

I don't think the model of a one lifetime fee really helps users. Because after one year, they don't have support or updates. And no support means a really bad experience.

You can't charge someone a one lifetime fee for an ongoing service. This makes no sense.

So the answer is SaaS.
Users pay yearly (or monthly) as long as they use the plugin or stop whenever they want. They are free to keep using the service or stop it. As long as they pay, they have support, updates.

I conclude on an interesting post from Chris Lema about this form last week :