Veteran pro wrestling carnival barker Jim Cornette talked to FanHouse (HT Cage Side Seats):

"MMA and the UFC have taken all of the pro wrestling fans because it's pro wrestling from 30 years ago, just in an Octagon and the fights happen to be real. But they're marketed exactly the same way. People want new stars, young athletes, a more serious approach. Athletic competition with interesting personalities with a hard sports edge and take the UFC fans back or at least entice a few of them to come back and then you'd have something."

"No one wants to try that because it would mean that everybody mostly over 40 in this business would be out of a job and those are the guys sucking up all the money."

The Camel Clutch Blog argues that the UFC isn't hurting pro wrestling (emphasis mine):

Ever since the UFC exploded in 2005, many pundits jumped on the bandwagon and predicted the demise of pro wrestling. Personally, I never quite understood the crossover and still don't understand why some pro wrestling news websites carry MMA news. I think the audiences are entirely different. I think most of the crossover comes from old wrestling fans that stopped following wrestling years ago and found something new to latch onto. As successful as Brock Lesnar has been in the UFC, I don't think it has as much to do with the pro wrestling crossover as people think. If that was the case, than Bobby Lashley's fights would be doing huge numbers, and Lesnar's Dynamite!! USA fight would have done record ratings on Showtime. At the end of the day I think these two audiences have as much in common as football and hockey fans.

Black Lesnar's informal survey of BElitists seems to confirm that a lot of MMA fans are former WWE fans.

But Dave Meltzer pointed out a while back that there's a clear correlation in good numbers for the WWE and UFC events:

WWE PPVs do 10% less than they "should" if they take place the night after a UFC PPV, and "even show some weakness" if there's UFC PPV 8 days before. This most noticeably affected Survivor Series '09 and Wrestlemania 26. Wrestlemania did 20% less than "the wrestling side" of the company did predicted that it would be, and 12% less than it was budgeted to do at the end of last year.

The WWE is apparently looking for hard evidence that they are NOT losing PPV customers to the UFC, via Pro Wrestling Torch:

WWE sent out a survey to WWE fans today looking to collect data on their PPV viewing habits after Sunday's Money in the Bank PPV.

The survey includes a question on whether fans watch WWE PPVs in groups, which has been WWE's explanation to shareholders for declining PPV buys.

WWE also asked whether fans watch other PPV programming, including naming TNA and UFC PPV events. WWE is apparently looking for evidence to support their claim to shareholders that there isn't significant cross-over between the WWE and UFC PPV-buying audience.

I really have to beg to differ with anyone who thinks that UFC heavyweight champ Brock Lesnar's incredible success isn't partly a result of him pulling his old WWE fans along with him. From Meltzer:

Lesnar's fights brought a new audience to UFC broadcasts, with company officials estimating close to half of the audience for his debut fight had never purchased a UFC event before.

Per Dave Meltzer, the UFC 116 PPV did very well but underperformed in the usual MMA hotspots and over-performed in the usual WWE strongholds. To me that indicates that the UFC is pulling in many still current WWE viewers. With money being tight, it's no wonder the WWE is losing sales.

I'd say the UFC has about wrung the WWE orange dry. The next step is to continue making inroads into the boxing audience. That's where the Kimbo Slice play was so brilliant. They parlayed Kimbo's fame and drawing power in the African-American community to push Rampage Jackson and Rashad Evans. I'd kill to know how many of the million plus buyers for UFC 114 were African-American and how many of those new fans are primarily boxing fans.

HBO has claimed that less than 5% of its boxing PPV customers also order UFC events. To me that means there is a huge audience out there to be tapped. Many of those boxing fans are African-American or Hispanic, but many are also older whites. The UFC 118 "freak show" fight between James Toney and Randy Couture should draw unusual interest from boxing fans in all three categories.

As long as professional wrestling is utterly controlled by the WWE and the WWE is a one-man dictatorship, pro wrestling will continue to decline. Zack Arnold has long argued that the collapse of Japanese proresu was a big factor in the later collapse of Japanese MMA because when proresu stopped producing dynamic new talents, MMA wasn't able to produce equivalent Japanese stars on its own. It's clear the WWE will probably never produce another Brock Lesnar so the UFC will have to grow its own stars from here on in. So far so good.