> one time charge, this one is tricky as restricting access requires a back-end that needs ongoing maintenance and server costs

If a browser extension is allowed to use license keys (not sure on the various store rules i.r.t. browser extensions), you could create a timed license key that is cryptographically signed.

No back-end required for that.

It is not clear what problem you are solving.

Extension devs know the rules of the game up front and have no expectation of profit.

Yes, most extension devs probably start out with no intentions of profit. They wrote their extension to scratch an itch. However, once they get an installed base and start getting offers to do shady stuff, it seems obvious that they might be tempted by easy money. If they had a more legitimate way to make money, they may be less tempted by the shady stuff.

That is indeed obvious but I'd argue that in this case the problem is weak moral spine. Fortifying it with money will not make it go away.

I may have misunderstood your prior comment so please excuse me if I got it wrong. The problem I was solving was how to make money from an extension that I publish. I was outlining different business models where you could give the user access to the extension, and make money without having to accept this arrangement with data thieves.

I don't see this as problem that needs to be solved. It is freeware from the start.

It's wonderful that people are willing to share their knowledge and time for free - why not let it be the way it is?

Perverse incentives being one, you make the extension for free, for the purpose of selling out later.

Project being maintained by a single dev being another, there needs to be incentive to keep the project going and not abondon or sell out.

I believe problem isn't the right word. I think OP is challenging your assumption that it's inherently freeware. There are methods for monetizing an extension and they're infrequently used or associated with a much larger experience (e.g. my BitWarden extension is critical for using BitWarden, but I pay for BitWarden's subscription elsewhere).