I know this is going to wander all over the place, but here it is anyways. My apologies in advance.......
While the analysis provided by CLSteve and others is well done and probably very accurate, there are some things that have been overlooked IMHO. First, people are concerned about the parks being maxed out, but they continue to fail to realize that the E-ticket rides are maxed out, and some of the family oriented B/C-ticket rides are maxed out. There are many attractions that have 5-15 minute waits for most of the year (yes, even in summer), they just aren't the "big ones". That part of the crowd is the part that Disney competes with Universal on. The thrill seekers. They are mostly interested in the next big thrill or doing only the big thrill's. And that's fine. But walking through many of these Orlando parks for better than 35 years, I can tell you that isn't the market Disney is worried about.
They keep some of those rides around, and will always in vest in one every few years because they become the marquee attractions that sell the location. However, it is the other attractions that keep the families and couples coming to the parks, and while the enjoy the big rides, they also enjoy the shows, the shops the atmosphere and the smaller rides. And those aren't at capacity outside of MK. HS, with all of it's attraction closings recently is probably near its current capacity for attendance, but it is still roughly 55% of MK attendance, and the park is almost the same physical size. If the choice is to build out the current parks or build a 5th gate, the 5th gate won't happen for decades. The start-up labor costs for tickets, brochures, information learned by all employee's, theme building, staffing all of the necessary things in a new park (
stroller rentals, etc.) is just too overwhelming when there is still capacity in their current slate of parks.
EPCOT for crying out loud doesn't even hit 1/3 of capacity for the space it takes up, but Soarin and Test Track have ridiculous lines. The rest of the park is fairly empty really. Try going to some of the shows in the countries - they are basically 0 wait time for 90% of the year.
Can more be done with the current parks? Absolutely. I have been, and will continue to promote the idea that WDW is shooting itself in the foot with its current park offerings for future customers. People go and spend their money, but the number of people willing to be repeat customers is dwindling. The number of friends that I have that went once and vocally talk about how they will never go back due to the cost and the lines. Those are lost future customers, and potentially, so are their children. Running a destination park requires a constant short and long-term mentality and view on all decisions.
Is Disney raking in the money now? Absolutely. But they are going to have a problem in the future if they don't start creating a more even experience for their customers at all of their parks. MK is still the marquee park from an attendance and attractions perspective, and it isn't even close. If you want to spread people to the other parks, you have to make it worth it to them to go. They will never change the status, but they might get people to spread around their vacation time, or heaven forbid, get them to come back year after year to see the things they missed the first time around.
I agree with CLSteve that there is no 5th gate in the near future, and that the staffing of the parks is a staggering problem to deal with (lots of low wage jobs, very little in the way of high-wage counterparts). However, I think that augmenting what they are already offering to get people to see more attractions will also help to spread around the love to the other parks and may help people spend their money in a variety of new and different ways. There are only so many MK and Mickey shirts you can own, so you need to get people into other parks so that they can see what else is available - both in attractions but also products.
TDO needs to realize (and they may) that Universal is not their direct competition (they are a little bit, but let's move on for a moment). Their direct competition is their own future visitors. You can't leverage the idea of always just capturing the new visitor and never work towards cultivating a repeat customer. Some will always come back (like me), but there are a lot that don't. I would be curious to know (Disney probably knows) what percentage of their customers only come once or twice a generation versus those that come every 1-3 years. I would vie for the latter customer as I can maintain my resort destination with them over the long term. New visitors will stop coming at some point (due to price, lines, lack of new things, etc.). TDO needs a longer viewpoint in their decision making process for the parks. I think they have adopted a good idea with the hotels (DVC actually pays for the operation of the hotel up front with fee's, so there is no cost to TDO to have them operating on grounds). They can't only have DVC, but they seem to be working towards finding a balance between the two types.