Upgrading from 1.4.0 to 2.0.0 was kind of a pain. I definitely am on the same page as you. Sellers attach a version number (of their own invention) to the items they upload. When a seller comes up with an update to their item they would upload it and update the version. At which point anyway who has purchased it in the past would be able to upgrade for free. (I don't like the idea of buying a theme and then paying to get the latest changes.)