The fragmentation is due to corporate greed not it being open source. They'd rather sell the next version of their phone than update all the previous versions to the latest Android. Realistically if they updated the older devices many people would lose the incentive to upgrade to a new phone because the end user experience wouldn't change as much.
It's their decision though, that's what I'm saying. No matter if it's a corporation or a single user. Anybody can download AOSP today and put it on their phone. Instant fragmentation (sort of).
Apple would be the same way if they opened up iOS. I never thought it was really a valid argument. I always thought the best part about iOS was the fact that everything was uniform from the apps, settings, and main interface. That's something Android will never accomplish.
But worrying about if your phone's not getting a timely update when it has to go through carriers, corporations, and different networks is just gonna have to be the case unless Google closes Android up.