It actually can't work. You have limited resources with potentially limitless spending. Free hosting isn't sustainable either unless you have P2H or ads, both of which make ongoing money. With one-time payments, there is no ongoing funding. You can certainly sustain service for quite some time, but you simply can't sustain it forever. At some point, the host is going to fold, due to lack of funding. It doesn't really matter how many people leave, as you still have fixed costs, and why would somebody migrate to another paid package, when they already paid you? There should be no reason for them to migrate to another package. That kinda defeats the purpose of a one-time fee.