I think that this will eventually happen because the parks aren't getting any larger, nor are they building new parks in the USA, but the population continues to grow.
Further, if they were to try to build a third resorts in, say, Texas, which would make the most sense, the environmentalists/anti-capitalists would go nuts, protest, and sue. Also, it'd cost them a ton more because there are all sorts of new regulations and hoops that they'd have to jump through.
What we'll invariably end up with is a very exclusive, population percentage-wise, family resort. The rest of us will hit Six Flags and the like.
WDW is on the edge of being out of my price range. I used to frequent it but over the last year or two I've noticed the dining going from "Ok, that was a bit more expensive but we're at WDW" to "Now I'm just feeling dumb paying for it." Everyone, obviously, has their own threshold for this but, if I return, I'll be dining offsite. Disney dining is both expensive and I have to plan for it MONTHS in advance. Their food, I think, is generally pretty good but it's not 4-5 star prices good.