Exactly this. I bought Nova 10 years ago (December 2015), and I even paid only 10 cent (Google had some absurd sales on apps back then). Till today, I'm using it, receiving updates, and never paid another cent to the dev AFAIK. I don't know how they even financed their business, but even as popular as it was (is?), I doubt they are swimming in money.
I can understand why so many apps end up in subscription-traps, or are selling the whole business to someone else. But at the same time it's insane that as a customer it's really hard to find a useful app, and have a way to continue support on a low level. For example, I probably wouldn't have a problem with paying Nova some dollar every time I switched to a new device, to get a new compatible version. But I can't even do that. There is barely any mobile app iterating this way through the android-versions. So they all are either dying at some point, or end in a trap.