I dont seriously think MMA is a threat to boxings popularity; I personally regard MMA as a joke - its more akin to american wrestling than a proper sport, and the appeal is just a sick kind of voyeurism to see masses of blood etc, rather than to see which competitor can his build, brain and tecnhique to outwit his foe. MMA is also mostly hugely boring imo - couple of guys cuddling on the deck like a pair of homos; Ive seen a few MMA bouts and its either cuddling, or someone being held down and punched in the head while defenceless, like a school yard fight -there is absolutely no honour in it. You can tell it sells itself solely on bloodlust, as opposed to thrilling competition, as the rounds are a ridiculous 5 minutes long and the ref never stops anything unless someone is literally at deaths door.
However, a lot of guys on this thread have made valid points about the value for money fans get from PPV fights. PPV fights are rare in the UK. Apart from domestic boxing, we normally get a lot of live US (east coast) boxing on subscription sports channels, and the only PPV over and above that is if someone popular like Hatton is fighting in a big contest.
The problem with boxing is that everybody wants a piece of the action, and this is the reason for the huge number of worthless titles and the increasing amts of PPV USA fans are asked to fork out for. Poor value for money on PPV and multiple corrupt boxing organisations are the main threats to boxing, not MMA. It is ironic that boxings huge popularity, (which gives rise to alphabet titles and conflicting PPV bouts etc), can at times be indirectly one of the main things to damage its image.
As for the future of MMA, I would be very surprised if it ever got much bigger than it is just now, (certainly in the UK anyway), which is nothing more than an occasional late night slot on an obscure cable channel.