>old users feel betrayed for having to pay the full upgrade price even though they bought the app earlier than new users.

Do they, though? Back when things like word processors were hundreds of dollars for the initial purchase and maybe half that for upgrades, it kept your users relatively happy. But when Apple introduced Pages on the app store originally, it was something like $30-40 for new versions. So I actually ended up spending less on it than I had on other word processors, but getting the same regular updates. It's free now, but I personally love the model of low reasonable price each time rather than really high first time price, and kind of high additional price when new versions come out. That initial high price meant there were a lot of programs I didn't buy when I wasn't making quite the salary I am today.