By the way, I wondering if it could be a new feature for premium, analogy to anki, anki could set your own sync server if you build one for yourself. Same as here, if you buy a life-long payment, this kind of feature would be useful, because it certainly is a guarantee that you could use it forever. Don't take this in a wrong way and respectfully , for a customer or user, an actionable method you can do to retain the product you've paid would be more convincing than a verbal promise.
But I wonder if there are some difficulties to achieve it, like someone would use it to copy your product, which would not be a good thing to happen.