I think I had this discussion in another thread, but exactly.
Most people aren't so close to Orlando that they can simply get to a park at any moment within a few hours. And most people have limited vacation days. EU might convince some people that didn't want to go to Orlando to go there, and thus also go to wdw while they're at it. But for a lot of people, one more offering means decreasing the time they were already going to spend elsewhere in their limited vacation time, to add in EU. And if EU is smart and sells people on the idea on sticking around for the other parks now that you're there for EU, wdw park days are going to be "sacrificed" to allot more Universal time. People that maybe went twice to each disney park maybe decreasing to 1 day only for each, and people who were already doing that will start not going to their least favorite disney park, especially since there's been little new stuff
Edit: and money, most people also have a limit to how much they can or are willing to spend
Disney getting complacent, thinking EU won't affect them will bite them in the rear in the long run