Comment by ibejoeb

I don't think the notion of subscription is off-putting. It's just not a very natural fit for something that isn't a function of time or resources. This is just a better model for upgrades. If you make improvements later on, people will pay again.

For this particular situation, your risk probably isn't that people will stop paying. It's more likely that people like it enough that a free alternative pops up (it's not so different from rectangle and alttab.) You're probably better off taking the money up front.