> get updates for X years, sync with all your devices and use it forever. If you want updates after X years buy the newest version again. If its good enough as is - and that's the goal - just keep using it forever.

While this sounds good deal, with this approach

- You have to charge total cost of subscription at once (1y or 2y),

- Still have to keep servers running for syncing, also you have think about cases where user syncing 1y of data in a single day.

- Have to keep people on the payroll for future developments.

(You are here thinking only in developer perspective.)