The number of new and repeat guests would justify the cost of building a fifth park. Frankly, it's the only way they're going to see a significant increase in attendance. New attractions will bring in more guests to the parks where they're built, but a new park would give people reasons to stay additional days. One thing people fail to realize is the impact of international guests. Many guests from Europe come to Disney World only and can take 2 week vacations. I've read some of their complaints about how WDW doesn't have enough to offer them. The additions to DAK may attract guests who willing to pay premium prices, but it won't have that much affect on park attendance.
Disney doesn't care about raw attendance numbers. Profit is the most important figure.
The problem with adding a fifth park is that it wouldn't cause overall visits to increase by a proportionate figure. When Disney opened the Animal Kingdom, they had hoped it would result in longer stays and increased spending. What they found was that--by and large--guests did not extend their trips into extra days to visit DAK. Instead they shortened their visits to other parks in order to add DAK.
The net result was that Disney pays millions of dollars per year to staff and maintain ticket booths, shops, restaurants, Guest Relations, parking lot attendants, custodians and dozens of other positions at that 4th gate, but they didn't get a corresponding increase in profits. People didn't spend more on their trips--they just shifted dollars from one park to another, one restaurant to another, one shop to another.
You're right--some people do come from overseas and visit for 2 weeks. The problem is that opening a fifth park wouldn't prompt them to stay for 2.5 weeks. Thus Disney's revenues don't go up while their expenses do increase to support a 5th gate.
Frankly anyone who is a Disney parks fan should hope that they do NOT open a 5th park. Developing a new park would require hundreds-of-millions of dollars invested in infrastructure alone. Before they add a single ride they need to bring in utilities, pave parking lots, theme and landscape the area. They need to build ticket windows, turnstiles, shops, restrooms, bus stops and restaurants.
And that's not even getting into issues with the labor market. Disney already employs 60,000 - 70,000 people in Central Florida. Do we really want them scraping the barrel further to add another 5-10k?
It's fun to dream of what a 5th park might look like, but in most respects it's a colossal waste of money. Better to spend that money on new attractions or "lands" in the four existing parks. I don't want a reason to venture out to another corner of WDW property for a new half-day park. I want Hollywood Studios and DAK to be parks that can hold my attention for an entire day.
Instead of spending $2 Billion to build infrastructure + 8 attractions at a new park, spend 1/3 of that and give me 2 new attractions at each of the existing parks.