I rarely want any service with the app that I buy. When I want it, I am happy to pay for it, too. But I don't want to pay for a fact that developer keeps tuning the app all the time. I pick a version and buy it. That's it. I will buy a new version if I want to sometime. If you offer me a discount if I "return" the old version when buying a new one - that's fine. But I don't want to enslave anybody to sit on producing updates that I do not need.

Also an idea about pricing the open source software: if you are driving in France past a sunflower field, you will often see a poster welcoming you to pick a stem and leave 2eur in a box for it. Tips appreciated, but 2 eur is specified. Why not apply the same to open source software? Plus a discount for the ones just upgrading from an older version.