My guess is that people tend to buy software, play with it a while, shrug, and move on. If a substantial percentage of those people, in the glow of a new toy, pay for the forever subscription, then those people are more profitable without much cost--either actual cost or opportunity cost. And if those people come back two years later, are they going to remember the forever subscription and do the necessary research to find their proof of payment, or are they just going to pay again? I can't figure out how I paid for OmniFocus; when I upgrade I'm just going to buy a new copy. This is all a guess.