I'd 100% push people towards offering an extension for free to build up a good amount of reviews & user base, then switching to any paid model (One off, Monthly Subscription or Annual Subscription). If people are happy to pay for something that was free, it's a good indictor that you've built something decent. I went for Annual Subscription because I felt it offered a fair price point ( < $5 a year per a user ) and it created an incentive for me to keep maintaining the extension. I'd recommend playing around and seeing what works best for your extension.

> Do you see any practical risk with someone offering a copied version of the extension

They're are already a bunch of extensions doing something similar for free. I like to think people have used my extension because I'm never more then an email away if they have any questions. Also as it's paid, I have no incentive to sell it to a 3rd party who might start injecting ads into webpages or something.