How I will have to pay for such an app (a one-time, lifetime subscription or every year) is the least relevant part for me. If the app does what is supposed to do, I will pay for it, yearly or one-time, it doesn’t really matter.
My actual concern is if the app will really be a good replacement for FlightTrack. As I wrote here previously, after FlightTrack’s demise I tried pretty much every app out there and none was even close to FlightTrack. All were a waste of money, as I deleted them after a few days. The only app that, in my opinion: 1.came closer to what FlghtTrack used to do and 2. was not cluttered with lots of useless stuff still is FlightUpdatePro. However, the developer(s) of FlightUpdatePro promised to listen to feedback and tweak the app to get it where it needed to be. This happened up to a point and then nothing. I really hope this will not be the case with Flightyapp but we will have to see. I cannot wait to play with the beta version.