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.
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.