When you say normal product, do you mean users will have to buy the next version?

We should be responsible developers with the ability to patch insecurities in the software we sell. You're also going to have feature requests you'd like to publish to your users. As a sole developer one evergreen version seems much easier to maintain for both these concerns. And I think a subscription model or some recurring payment model makes more sense both for you and your users. Explaining this to your users maybe difficult, but if they actually find a product useful they should want the developer behind it to be able to support it.