The best model I know is the one used by Sketch (https://sketchapp.com/pricing/):

  A year of updates.
  A lifetime of Sketch

  Your license comes with a year’s worth of updates.
  Once that year is up, you can keep using the last 
  version of Sketch you downloaded, forever.

* You can always open the files you have made

* There is no incentive for feature creep caused by having to "sell" the next version to your current userbase every so often.

* The developer does not have to keep supporting old versions forever.