What is the most useless app that you have seen being given as a subscription?
For me, I tried a 'minimalist' launcher app for Android that had a 7 day trial or something and they had a yearly subscription based model for it. I was aghast. I would literally expect the app to blow my mind and do everything one can assume to go that way. In a world, where Nova Launcher (Yes, I know it has been acquired by Branch folks but it still is a sturdy one) or Niagara exist plus many alternatives including minimalist ones on F Droid, the dev must be releasing revolutionary stuff to factor in a subscription service.
Second, is a controversial choice, since it's free tier is quite good and people like it so much. But, Pocketcasts. I checked it's yearly price the other day, and boy, in my country, I can subscribe to Google Play Pass, YouTube Premium and Spotify and still have money left before I hit the ceiling what Pocketcasts is asking for paid upgrade.
Also, what are your views on one time purchase vs subscriptions? Personally, I find it much easier to purchase, if it's good enough even if it was piratable, something if it is a one time purchase rather than repetitive.
Specially in mobile, if you don't update your app yearly, it will probably be removed from the store soon. Google and Apple can randomly Review your app and decide that it failed review even if it passed in the past. And fixing it to pass review is often not trivial and can take weeks of work.
Also, with each new version of Android and iOS, your app can stop working or become outdated. The platform API changes frequently.
Finally, if you use any third party libraries in your app, vulnerabilities might be found in the that you'll want to have patched ASAP.
Oh and of course, you need to pay 99$ a year for Appstore access.