Nostalgia is still a big draw I think - baby boomers and otherswho visited the parks back in the 80's and 90's want to bring their kids and grandkids because they remember how awesome it was. To them it's kind of a rite of passage. I do wonder how many of them keep returning though now that you need a personal trainer to plan your magical experience for you. Then there are those who never went back in its prime and don't know any better because they have nothing to compare it to so they just buy into all the pixie-dusted c r a p. We have a neighbor here like that. He always wanted to go when he was a kid, but his parents couldn't afford it, so this is his mecca. He's been 3 times in the past 3 years and can't wait to go back...well except for the fact that he and his family all got sick on the food during their last visit....
There is another factor involved which makes the downfall of Disney less then a sure thing. That is that most of the upset is coming from those that were there 30 to 35 years ago, mostly as children or young adults. I would be one of those. I miss the old Epcot a lot, but, I am not that upset with what is currently there, but as for the rest of the place, I have no real issue with it and in many ways I am sure that I see more vibrant colors, nicer looking areas and more flowers and beautiful landscaping now then I remember back then. I can tell, that as I age I go for less time and less often simply because unlike before, sometimes there is pain all mixed in with my enjoyment. In other words, we are much less relevant to their success then we were in our youth. We are naturally fading off into the sunset.
But the key factor is that as
@George said, as we have gained more money, fewer small children and the aches and pains of aging, our personal interest level is lessened. Our tastes and desires have altered over those years and are spending money on other things. And then there is that ever looming "Bucket List".
New people arriving now, do not have those memories, real or imagined, of what it was like years ago. They see WDW for what it is now and it is very impressive to them, just like it was to us when we first set foot on property. That first impression coupled with the fact that they have no real basis of comparison, will keep them coming back. Their outlook will be the same as ours, only without the "I remember as a child" burden attached. That will come later, when they are not as relevant either.
It appears that Universal is taking days away from Disney and yet Disney is always full of people. Maybe the idea that there are two world class theme parks to go to in Orlando, is bringing more in then before. I have always enjoyed Uni and have fairly regularly spent days there since it opened. I am not locked into a resort because I stay offsite for either of them, so the number of days I spend at Disney is determined by how much time I have available along with what interests me at that time. I love having choices and by the looks of things no matter what my decision, no one seems to be hurting because of my decisions.
Nostalgia still exists and is what is bringing all of us ancients back, but, it is not a factor for the newbie. They are currently creating what will be their nostalgia. They know of no cutbacks, they know of no lessening of quality. Disney does just enough to hide most of those that are not looking for it. To them there is nothing out of line.
Does that mean the end of Disney? Not by a long-shot, because to this point, they have managed to remain a pretty impressive place. And, I might add, it hasn't been because of us complaining. They are not Walt Disney, but, they mostly do know what it takes to remain world class. We may not like it much, but, others apparently do.
I'm right in that boat with you, I nearly hit the ceiling over it and can't wait to see what Universal comes up with. Exciting times! 
