Stability updates to deal with API changes, perhaps. New features like widgets? I'm not so sure.
I've paid for an MP3 player app six years ago and the APK still works fine, although the UI looks a bit wonky on a phone screen four times the size of what I had back in the day. I don't expect anything more than an update in case MP3 playback somehow breaks.
Maybe this is different in recent years, with iOS putting the burden of backwards compatibility more with the developers and Google now following suit in their attempts to add decent privacy restrictions to a previously unrestricted API, but I think consumers don't expect all that much from one-time purchases.
You'll lose market share if you don't add new features, as well as new customers if your app is falling behind others, but that's a separate problem.
On the other hand, the moment you ask just a single euro per year as a subscription, people expect to be up to date all the time. After all, you're still charging them even, so why wouldn't the app ever improve?
Everyone knows a software business does better with recurring revenue instead of single purchases, but customer expectations are also higher if they're still paying you five years down the line.