Lastly, the idea that HP didn't cause WDW to "lose anything" based on attendance numbers is just a non starter. If Universal's numbers didn't jump by 30 percent, chances are WDW's would have been much higher - probably not near 30 percent but possibly 10-12 percent (just throwing my own arbitrary figure out there). Income lost is income lost and believe me Disney knows this.
As of right now, I disagree.
It all depends on how the market as a whole has grown and/or shifted. If Wendy's sees an increase in their revenues, it doesn't necessarily mean that McDonald's has lost anything. Nor does it mean that McDonald's had any shot a taking those gains away from Wendy's.
In this case, if people were coming to Orlando specifically for WWoHP and have no history of visiting WDW, there is no reason to believe Disney could have gained that business instead of US. (Aside from Disney having contracted for HP in the first place, of course.)
The other likely scenario is that Disney has itself benefitted from the presence of WWoHP (and Legoland, and others.) If WWoHP is the draw, chances are some of that business spilled over into Disney's parks. In other words, family goes to central FL to see Harry Potter, but also decides to spend a day at the Magic Kingdom since they've already in town. Some will get multi-day passes and visit multiple Disney parks.
It's hard to draw any firm conclusions--even uneducated ones--because of the state of the economy over the last 4 years. Universal's numbers look great when expressed as a percentage but reality is IOA still draws 2 million fewer guests per year than either Animal Kingdom or Hollywood Studios. Universal Florida is 3.5 million under those two parks.
Disney's 1-2% increases don't look all that great but in the wake of record unemployment and underemployment, any gains at all may be laudable. And Disney's average guest spending has consistently risen.
That said, I do think Disney has reason to worry in the coming years. Guests taking a one-day pit stop at IOA for WWoHP may have a negligible impact. But, when more HP is added to the USF park combined with Transformers, Despicable Me and value-priced hotel rooms....suddenly Disney could be faced with losing guests entirely. Instead of staying on Disney property and popping-over to USF for a day, they could find more and more people staying at a Universal hotel and making day trips to Epcot and the Magic Kingdom. That hurts hotel occupancy, restaurant revenue, park admissions per guest, etc.