Its not even just the parks, literally the Anaheim area surrounding Disneyland (which you constantly have to deal with to get in and out of the parks, unless you are staying at one of the few, very expensive "on site" hotels) is already one of the most well-known traffic nightmares in the nation. Some would claim it already is at crisis level - and SWL is going to just obliterate any sense of normalcy.
Then you factor in that Anaheim isn't "isolated" like Orlando, and millions and millions of people come to the greater Los Angeles area every day for reasons that have nothing whatsoever to do with theme parks or even a vacation, who will be buying one-day tickets just to go see this new Star Wars land. Assuredly, though, for various reasons, it will spill out - people like "I'm not waiting six hours to do a ride, let's go to the rest of Disneyland..." or those that can't ride/have little ones who will just come to walk around and then spend the day at Disneyland.
I honestly don't think people are being crazy when they say how unprecedented this is going to be. Not just in terms of theme park demand, but overall demand for just access to a particular public place. I think it is even possible Disneyland will be going 24/7, one way or another (hard ticket all night multiple events every night, or just staying open) quite beyond their historical norms.
At least WDW has somewhat of a self-controlling entity - hotels book up, virtually everyone who is coming is coming for an actual block of time, but Disneyland? I mean, they undoubtedly are going to close to capacity daily for the foreseeable future after SWL opens. Estimates are, Disneyland has about a max capacity of 80,000 - and it's gonna be a new level of public demand for any attraction anywhere, and I won't be surprised if on any given day you have 200,000 people trying to get into that area, and it's going to go on for a very long time, at least until Orlando opens, and probably long after.