For every couple year major versions (with decently advanced notice it's coming and/or a recency allowance) I think it's fair to charge anew but for minor improvements over the lifecycle of a major version I think it's fair to want to buy everything that will come in the major version up front if you're making a one time purchase. Tons of software is this way from Sublime to Windows to ZBrush to an absolute crapload of games. In this model a one time purchase is seen more as an alternative to holding a subscription over the lifetime rather than the intent to forego any future features or enhancements out of interest of the exact current feature set being all you'll ever want. There is also the "in-between model" e.g. IntelliJ where you get the current version + a period of updates and you can either stay with where it ends at that period or pay a smaller amount for more updates.

I'm not sure which group the Halide changes mentioned above fell into but just on the general topic I think it's a fair expectation.