Six years of running a period tracker app with no subscription, no ads

I've been running Simpluna, a period-tracking app, for six years. It's a company based in Tokyo, Japan, but development and operations are basically a one-person effort. The app is built with React Native (Expo).The defining trait of the app is that it's simple, with no unnecessary features. Recording periods, calculating predictions, and sending notifications — that's the core of it. This space is dominated by big players (Flo, Clue, and in Japan, Luna Luna). Building something similar would have been hopeless, so I made the things those apps don't have my selling point.Optional features beyond the basics are offered as one-time purchases. Most apps in this space use subscriptions, but I didn't want to. Subscriptions make revenue easier, but they're a mechanism that keeps manufacturing reasons not to cancel, and features keep piling up as a result. By staying away from subscriptions, I could keep the app simple — a tool that doesn't intrude on the user's life.With privacy in mind, data is stored locally on the device by default rather than in the cloud.So far, the app has found a solid footing in East Asia — Japan, Korea, Taiwan, and China. It has almost no name recognition in the West or elsewhere, but there is a small number of users in those regions who like it and keep using it.The reality, though: the app is profitable and covers its running costs, but it's not yet at a level that supports a living, so I have no choice but to keep it going alongside other work. I see this as one of the costs of not taking the subscription route.I have two questions. How do you see the difficulty of sustaining an app on one-time purchases alone? I'd love to hear from people who made the same choice, or who gave up and moved to subscriptions. The other: could an app like this be accepted in North America and Europe?