Minor nitpick, compared to it not being open source, so it's not an ideal one to begin with. There's no reason a nearly-identical system couldn't be used as a paid-upgrade tool; just download the license file that was purchased in-app, and have the newly-updated application read it in.

It was chosen for conceptual design of something which has radically changed the application-updating playing field by being so successful. Normal people update regularly with Sparkle; how many have you seen ignore a Java / Windows / Adobe update for the thousandth time?