Personally I don’t like subscriptions very much.
I prefer to pay once, use it as long as I wish, and decide myself when I want a new (upgraded) version.
That said, I can’t imagine how 10 EUR once in a live time could be a sustainable business model.
Kurviger is not Whatsapp/Facebook. (there you are the product being sold, not the customer 
)
If we want Kurviger to survive we’d have to pay more. 
To attract new users, and don’t annoy existing users the current price model IMHO should be kept for the current feature list. But new features could cost extra.
An alternative to subscription may be plugins for new features. (OsmAnd is doing this AFAIK)
Possible candidates for extra payment would be:
Manfred