Literally every company in this economy currently, have you seen the price of a 2x4?
Disney has added enough new attractions recently at their existing parks to hold down a 5th gate by itself, not sure why so many get hot for a 5th gate, capacity wouldn't change even if you did take all the new attractions, and put them in their own park. Also adding a 5th gate would drive up popularity, so i can't imagine that would change much at all. In my mind adding a 3rd park in the US could start to break up the demand at WDW, but being Disney's largest park, it may not help a ton either.
I think you are putting a lot of the blame on Disney for being so popular, to me it comes off as people demanding to drop the capacity limits at the park so that they can have a more enjoyable time, the problem being people are more than willing to pay higher prices currently to just sit and wait in line (Not saying i agree with them, but it is what it is). There is a reason that Universal is more reasonable, it all boils down to supply and demand, and Universal unfortunately doesn't come close to matching up to Disney in that category