Exactly.
Getting people to stay longer is no longer a driving force in expansion, as in general they can't. In 21st Century Corporate America, you don't get as much vacation, and companies also don't want it accumulating either as it is a liability on the books.
The driving factor in a 5th gate will be capacity - when they can no longer reasonably accommodate the guest load across the four parks they have. And then it is an enormous expense for what would be little gain in revenue. It comes down to timing...too early and profits fall through the floor. Too late and you're losing revenue as guests decide not to come because they can't get into the parks any more...and then you won't need it as guest attendance will drop through the floor...
As reviled as Eisner is in some circles, he did have foresight to know what they were going to need, and got it built before it was too late. No matter what, each one was a gamble, and he probably finally lost the pot on the last one.
Mentality shifted away from the parks while the other corporate siblings struggled, and the parks were the tentpole. Whatever the parks made went to pay for ESPN and Studios losses. Now that those have turned around (despite some best efforts at the Studios at times), you'd think the parks would get more love, and so it started with the FLE - but then they went and made a deal for Shanghai and pushed forward a new tech initiative that involved computers and teh interwebs (something Disney has never seemed to get right to start with), and guess where all the money is going instead of planned expansions at the other parks?
If they can't pull off a Star Wars expansion or an already designed Cars Land, what hope is there for a whole fifth gate?
I have no doubt there are plans for a Villains park. I'm pretty sure there are many plans for a possible fifth park. I'm also willing to believe a Villains park is at the top of the list, and those in flashcal's Imagineering circles have been told as much. However what I cannot believe is that they are willing to commit the money to build it in the next 5 years. I believe the 20 year plan probably still includes it, but that's sort of "if we built to the max" document more than an actual roadmap.
And as I've said, given the backlash about the FLE they've already seen, they are very unlikely to build an entire WDW-scale park with a unary demographic focus. They will appeal to a more broad base, such that Villians would likely be a "land" within the park, perhaps the primary focus, but there will be other parts as well.
Oh, and about capacity being the driving force for an additional gate?
Disneyland is feeling that now. They've suspended sales of the SoCal passes because of the parks being too crowded. So after Shanghai's issues are dealt with, where do you think the next focus will be?