Almost every online service is using a subscription format, so I don’t think it would benefit them much to switch to a once off payment. Most apps are free and give you the option to subscribe if you feel you use it enough, which seems to be the most profitable thing to do these days. This is why most apps/games on the app stores are fremium/free to pay.
Anyway, if you use something daily, wouldn’t a monthly payment seem worth it?
If you live in Ireland the monthly payment is half minimum wage for one hour. If you live in Spain it is just over the minimum wage. If you live in Kosovo it is almost five times the minimum wage!
“Worth it” is very relative and we shouldn’t be surprised that with users from all around the world it might seem more expensive to some people than others 
And if you look in the app store you see most apps use a once off payment, so it makes sense that people would think about that as an option, even if it is not what language learning apps usually do. I think Memrise did a survey ages ago, before they released an app, asking how much people would be willing to pay for it as a once off purchase, but I might be misremembering the wording.
I’m just saying it’s a perfectly reasonable question, that’s all
Yeah ok, but there are the forums and the possibility to be trolled. That is currently happening to a lot of people. I recently had my first ads on Duoling app. One of them was “How to make $365 a day without doing anything. This ‘copy and paste’ method is far better than your job.”. … What a scam
$90 aus dollars per year… hmm… not quite worth it for me!
I know a lot of people who left Duolingo because they hated it after a (short) while: non-sensical sentences, all sort of constraints about what to learn etc
props to you for reviving this thread after three long years
welcome to the forum!