Don't let
@WDW1974 hear you say that!
I agree to an extent, a 5th realize gate does siphon off guests, but there is a whole lot of optimization that can and should be done before that is required (although I agree, in the current attendance trajectory it will be required).
The dollar for dollar figure (they would spent on a 5th gate) should first be spent fixing existing infrastructure and adding capacity to the three other parks that are not pulling their weight.
Animal Kingdom of course being the easiest to shift towards. Yes, they run the risk of making the park, which is on the lower end of capacity, too attractive. But, there are significant gains in capacity just by virtue of extending park hours. I think they are making very smart infrastructure moves already with by-passes, adding table service restaurants, preparing additional entertainment. They could still be bowled over, but at least there some preparedness going on.
Epcot is the other that has lots of capacity (and used to have more before the ceremonious stripping of several omni-movers). The main issue being the high capacity rides (Energy, Imagination, Living with the Land and to some extent Nemo) are its least compelling offerings.
DHS runs the risk of becoming a bit of a nightmare though if StarWars makes it too compelling, there is not a lot they can actually do to increase the footprint of that park.
Then there is Magic Kingdom, which quite frankly, does not need a new E-ticket. That would be about the worst thing you could do to that park right now. It must quietly and modestly increase its capacity while letting the other parks open the crowd pleasers. Even if we want them to build that Frozen attraction set for TDS in Magic Kingdom, it would push the park into complete nightmare territory. Yes the Magic Kingdom would finally have the E-ticket everyone has been begging for, but would the 2 million-extra-bodies annually be worth it?
To throw my support behind you about how new gates can be executed properly, some TDL figures...
1988: 13.38 million (5th Anniversary)
1998: 17.46 million (15th)
2008: 14.29 million (25th)
2013: 17.21 million (30th)
The key part being the opening of TDS properly alleviated the problem in 2001 by pulling in nearly 12 million guests in its first year. Last year TDS brought in just over 14 million. OLC smartly realizes there is some optimization with capacity at both parks still to be made with the mid-term projects, but long term that's the first resort that needs a new gate most imminently.