JetBrains does something similar. They give you access to all updates, including any major versions, for a year. (They put out a new major version approximately every 8 months, so depending on when you buy, you might end up with two major version upgrades for free.) Then you pay to maintain your subscription year after year, which you can think of as upgrade pricing.
Exactly :-) My point was that their business model is already very similar to what you described ("give me fixes for the version I purchased and then sell the next major version").
