Where will the amount of people needed to staff a 5th gate come from, particularly when the Orlando labor market is reportedly already tight and Universal is also building another park up the road?
There's also the land question. Yes, there is a lot of unused land, but a lot of it is unsuitable for development. Where would a 5th gate go? The one of the larger suitable chunks is south of DHS, near WWoS. (That would mean that the All Stars and POP would be the closest resorts and thus command a(n even bigger) premium. That just seems... wrong.) Would they spend the money necessary to make all of the land west of MK suitable for development? That would present even more questions around the transportation infrastructure in that area. There's another large chunk similar to the land west of MK out past 429 and Western Way, some suitable, some marginally suitable, so again, $$$ would be needed to make it all suitable for development.
Personally, I don't think the current management has the appetite to spend the money necessary to build a 5th gate in Florida. People can point to Shanghai but Disney did not foot the whole bill for it. $3.4 billion is what I read was the estimated bill for TWDC of the $5.5 billion project overall. It would likely cost far more than that to build a new 5th gate.