If someone can only take a seven-night vacation, travel takes out the first and last days. so six days at Disney, two at Magic Kingdom, one at each other park and one day at waterparks or Disney Springs or back to one of the other parks again. Add a 5th park and that guest is still going to have a seven night vacation, but they might spend the sixth day at the new park instead of a waterpark, the amount of revenue Disney gets is tiny compared to the cost of running the park.
Now on the people (like us Brits) doing a 14 night trip, an additional Disney park might result in me not going to SeaWorld or Busch Gardens, but I am still buying the same 14 day Disney ticket, I'm just saving £100 by not getting the SeaWorld tickets... Again, the only think Disney gain from me is one $20 lunch.