I don't know. 
It better not go away!
What is being observed in reference to PPV numbers may not be a 'downturn.' It is true to say PPV numbers are trending downward, but that is very short-term in comparison to the time spent gaining revenue. Empirically, it is normal behavior.
Thus far Zuffa has increased market share through a conservative fiscal policy, aggressive licensing, and innovative allocation of economic capital. Their ability to incite, and sustain growth seems to be independent from the vehicle that is used to facilitate that growth.
A network television deal may be the 'nail in the coffin' that you speak of, but I am not aware of the model you used for risk assessment.
I havnt looked at the totals. But is there actually a drop off in ppv buys? Like I know per ppv is probably down. But if you look at last year as a whole, then this year as a whole, are the numbers down? I can't believe they are if the UFC is expanding their amount of shows next year...
Also the amount of injuries to main events crushed the numbers this year I'm sure of it
And to answer the TUF question. I do believe it will hurt the UFC to not have a weekly show on fox to push this sport into the mainstream. If the events are monthly I think too much time will pass between shows to catch the attention span of the average couch potato into making a commitment to catch events 