As I said earlier, we're dealing with companies and as such, one has practically a monopoly on the upper-rung of the sport. I believe we can all concede that.
When you're dealing with companies who are not bound by a trade group, jurisdictions, or any type of trade group, they control the product and are able to feed it to the public with little or no question on whether or not the product is quality. A second viable entity would immediately cause the commanding company to examine its product and elevate its quality to maintain its market control.
Rivalries are based on history and growing a product from the ground up creates these situations.
I agree that I always want to see the best fight the best and I want it right now but that's a very short-sighted. What is far better for the sport is for two great fighters to build a legacy and for them to eventually face off. However, the current Zuffa model would never support that regardless of the concessions of another company to co-promote. I believe DFW and the Fertittas learned their lesson when their golden boy Chuck ate canvas in Japan and couldn't even earn a shot at Wanderlei.
I believe the Co Promotion went out the window after Dana and the Fertittas tried it again, this time in the UFC, even going so far as having Wandy in the octagon only to have have Pride him him from the fight against Chuck. It turned out they were just using the spot as a way to promote Wandy before hosting there on card in the States