I think you can argue this fight wasn't won in the ideal way with the towel being thrown in. I would think the third fight would be far bigger in terms of viewership/Pay-per view.
The alternative would possibly have been him finishing the fight as a twitching vegetable. It was a pretty emphatic victory. The first fight raised questions. This one answered them for good.
I'm sure a third fight would do fine. It wouldn't have anywhere near the same urgency and drama.