That was more true in the past because they had a built-in advantage of location... Chicago was the center of the universe and there wasn't enough critical mass anywhere else. That's an advantage, but Expo has seemed to use that as a crutch.

Vendors are not limited to Chicago... the biggest aren't even local. So they travel..
Stern Tour? We have the internet now... not nearly as lucrative
Industry people? Given the right circumstances.. they will travel too.
Collection of games? Expo has been surpassed, and at many locations
Major reveals? They can happen at other shows and have been...
Newer things like homebrew or start-up parts? They are happening elsewhere too

Expo is riding on inertia... while the elements that used to be unique are no longer. I doubt Expo will go away anytime soon, but it won't be the 'must be at event' it was in years prior. Product launches, travel, vendors, speakers, etc are all outside Expo's death grip and have been for awhile. Expo could shrink dramatically. I don't think the stronghold on appearances will hold up. Other shows will be able to invest to bring people in.

Honestly right now, I'd say the calendar is probably a bigger blocker for a competing show than Expo's presence. The scene is big enough (IMO) to support more than one show a year in the area... the problem is so much of the calendar is already busy with competing events. I think Expo's built in advantages are a thing of the past... and that's why vendors and individuals are able to switch to alternate shows and not see it as that huge of a loss.