The impact on other parks will be nutty IMHO.
WDW has it's share of local attendees, annual passholders, and guests who inhabit the parks multiple times per year. However, WDW is largely seen as a destination vacation. For many visitors, WDW is a long trek, a 10 day affair, or a once in lifetime vacation.
Star Wars is a HUGE draw, and it may be a catalyst for many visitors to start planning vacations. The truth is, people coming from New Zealand, India, and Wyoming won't JUST descend on Hollywood Studios, even if they have lightsabers and droids on the brain. They want to experience ALL the parks. Therefore, I believe that attendance will go up across the board.
Then, consider that
Disneyland just raised prices, and WDW is getting ready to raise prices for the second time in less than a year. Park attendance seems to be increasing, despite the price increase, and well before the inclusion of a land with a crazy fanbase. If overall park attendance is on the rise prior to a popular, new attraction, it seems logical that overall park attendance would continue to increase with the inclusion of a popular, new attraction.
WDW is building new hotels, adding new means of transportation, and making changes in regard to parking. I would argue that they are expecting a lot more guests than they used to expect. If not, all that stuff is a weird coincidence.
Lastly, some annual passholders talk about suspending their passes for a year or two after SWGE opens. Now, these people have a bit of WDW experience, and they won't necessarily avoid the parks for 2 years. They're just convinced that they will NOT get their money's worth out of annual passes during the initial Star Wars run. They're talking about everything being insanely crowded.
Don't get me wrong, I think you'll have a great time. I just think you have to be careful with expectations and planning. I'd recommend splurging on extras like DAH and EMM if you're able. The upcharge events may offset the large crowds you'll endure on a regular day.