Penn proved by landing a couple more punches than Hughes in the first and by failing a sub attempt in the second that he is the better fighter.
I think we can all agree that the remaining three rounds would have been moot even without the injury; having lost the first two rounds (I'm surprised those couple punches and failed sub didn't make the rounds 10-8, to be honest), it goes without saying that there would have been no way for Matt to win the remaining three. As far as I can recall, there has never been a case of a fighter legitimately winning a fight by taking over in the later rounds.
So why not just have one round? I think having more than that just makes it more likely that the inferior fighter might be able to eke out a win through conditioning, or that a fluke injury or slip might allow the inferior fighter to capitalize, and in either case we might see the more un-educated fans mistakenly assuming the winning fighter is actually the better fighter.
The point of the fight is to show who the better fighter is, and since only the early part of the fight can do that, shouldn't we limit fights to one five-minute round? Or maybe just a minute or two.