It’s no win scenario for developers. While you’re ok with “one up front payment and then maybe in app purchases to pay for new features going forward” there’s a whole other slew of voices who are going to complain about being nickled and dimed.
Given those choices (and other factors), as a consumer I prefer the subscription model. If nothing else, it lets me forecast my expenditure and continually re-assess the cost/value proposition of the application in question.
Disabling IPv4 isn’t going to do anything to move IPv6 forward. You’re just shutting those who remain limited to IPv4 through no fault of their own.