When you say the boost was "fleeting" it really depends how you look at it. Wizarding World opened in mid 2010. In 2011, according to the TEA numbers, Islands of Adventure saw a 29% attendance increase over 2010. In 2012 they say a 4% increase over 2011, which means they held onto the entire 29% increase and added 4% more which was the best percentage increase of any major Orlando park that year.
I would argue that that large increase also has something to do with the fact that Universal suffered huge losses in attendance the year before, each suffering a 10% or more drop in attendance. Regardless, no denying the expansions did cause impressive increases, but not as impressive as they may seem. This is why I hate statistics, and avoid them most of the time. In 2012 (just using that because I can't find the 2013 stats right now) Magic Kingdom saw a 2.5% increase, while Islands of Adventure saw nearly double that at a 4% increase. However, because of Disney's much higher attendance numbers, their 2.5% increase was an increase in attendance by 394,000, while IoA's 4% was only an increase of 307,000. So, Disney actually saw a larger increase in attendance than IoA, yet the statistics suggest otherwise. The statistics here are also skewed by a lot of other variables as well. Disney's mammoth attendance helps Universal, because as a previous poster mentioned, many visitors from Disney's numbers go to Universal, not because of Universal's expansions, but because they drove/flew all the way to Orlando to go to Disney and want to do everything while they are there. And the point I made earlier, Disney may be benefiting from Universal's expansions as well for the same reasons and viceversa, but the numbers are not likely anywhere near as high as Universal benefits from Disney.
Looking at the actual numbers, it still appears that over the past six years (which is as far back as I can find reports) IoA attendance has increased quite a bit higher than all other parks. Not as much as 2011 and 2010 would suggest, because of the significant decreases in attendance both parks took in the years before that, while Disney's parks didn't see nearly the decreases US and IoA saw. I added in those previous years, because an increase after a large decrease does skew the increase.
Increases in attendance from 2006-2012
Universal's Islands of Adventure: 2,681,000
Disney's Animal Kingdom: 1,088,000k
Disney's Magic Kingdom: 896,000
Disney's Hollywood Studios: 812,000
Epcot: 603,000
Universal Studios Orlando: 195,000
It will be interesting to see what increases we see from the New Fantasyland expansion when the 2013 numbers come out. I don't expect them to be as high as Wizarding World, but it will be interesting to see. What really surprises me is Universal Studio's increase being so low while IoA's is so high. IoA's increases do seem to have leveled off a bit, and seem to be very close to Disney's now. So the argument that expansions make a difference is a very credible one. It will be interesting to see what happens to attendance over the years now that there are no more Potter movies or books coming out.
I will throw out there that combining Disney's parks and Universal's, Disney still saw a 3,399,000 increase to Universal's 2,876,000 increase.
So, with all these numbers, let's be honest. No one is afraid of anyone. Everyone is swimming in their money without a care in the world.