You're using a device with an ever-evolving OS that constantly breaks compatibility.
If you paid for it once and only got to keep the version of it as of when you paid for it, in a couple years the platform would remove your ability to use it.
Don't want new features, and don't need an upgrade then stick with it. If you need the new version, then pay a partial amount of the app for the upgrade?
Does Apple not let you charge an upgrade fee?
Don't want new features, and don't need an upgrade then stick with it. If you need the new version, then pay a partial amount of the app for the upgrade?