Yes, that is a valid point! From my experience the card creating process is also really valuable and forces you to distill the information down to something concrete.
But I totally get you, that for something like learning a language it would be nice to have some predefined decks.
I would not pay for this but might buy it as an app on my phone.
I would be more likely to buy a subscription of you had a ton of decks on it like memrise does.