I have had subscription based apps in the past, none at the present time, so I am ok with the idea.
Obviously, a subscription will generates more revenue than a one time purchase, so I understand why it is prefered by Kosta and Ashley. And I think they deserve the money. I do wonder if the $3 a month charge may scare off some people, remembering the visually impaired community has about an 80% unemployment rate.
Personally, I have no money worries, and I will subscribe to the App to support the App, if for no other reason. But being a huge fan of FlickType classic, I wonder if others may be satisfied with the classic App.