Apps that shouldn't be Subscriptions
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.
Subscription only makes sense if there's an ongoing service, e.g. processing in the cloud, cloud data storage, etc.
Apps that don't need to be subscription:
I would say if you accept subscription services as justifiable, Duolingo is justified. What you’re raising is poor performance, not a reason for it to be purchase only.
Of course, I would be in favor of “the app is free, pay a set price for a language pack” rather than a subscription for premium.
I agree that a lot of subscriptions are really overpriced, but updates to an app are also a sort-of service. Pixelmator explained it quite well when their app switched to a subscription model, mentioning some fair (I think) pros and cons of the succession model, both from the perspective of users and developers.
https://www.pixelmator.com/blog/2022/08/18/why-pixelmator-photo-is-switching-to-subscription-pricing-and-a-sneak-peek-at-pixelmator-photo-for-mac/