I'm surprised to not see a lot of one-time payment web apps. With my side project - https://gifmemes.io (shameless plug), I've experimented with Google Adsense and in a month, it made on about 5 % of what I got from one-time watermark removal payments, so I've turned it off. I've thought about experimenting with subscription as well, but just found it too repulsive for that kind of project.