Personally, I don't think it's a matter of drawing people from WWOHP (and why do I think of IHOP every time I write that?)
I think most people don't do Disney (or Universal) every year. Some go every year, some go once in a lifetime, but I'd bet there are many people who go every couple of years. Something big like Star Wars might draw them to choose WDW that year instead of some other place.
At most (in terms of the effect on Universal), it might affect how people split their trips. Assuming many families go on vacation for a week, maybe they do an extra day at Disney because of Star Wars, and Universal loses a day. Or perhaps Sea World or a beach trip loses out altogether.
So, with the expansion at WWOHP and new stuff at DHS & AK, I'd expect attendance records at both parks to be broken, rather than one park stealing from the other.
What I'm wondering about is how Star Wars meshes with Avatar in terms of schedule. I've heard rumors of 2017 for both.
From a guest perspective, a massive rehab for DHS and AK in the same year could drastically affect WDW, in terms of attendance, crowds, touring plans, everything.
From Disney's perspective, it seems unnecessary. Rather than having two big things in the same summer, they'd be much better off spreading them out over a couple of years. Something like this:
2017 - Avatar land opens. Restaurants, shops, extended park hours, evening show. Maybe one new ride.
2018 - Star Wars Land opens. Massive new theme, cantina restaurant, etc. Maybe one ride.
2019- Avatar's biggest new e-ticket ride(s) open.
2020 - Star Wars Land's biggest new e-ticket ride(s) open.
Spread the excitement over several years, & you're likely to get more repeat visitors over the same time period. I know I'd fall for that.
