You are correct that Disney will continue to be #1. However, you can be less of a #1 than you currently are and see your profit margins shrink because of it. Disney makes the bulk of their money from the food, merchandise, etc that a guest spends. They want you to take their bus from the airport, stay in their hotels, and buy their meal plan so you won't consider anything else. If competition in the area continues to grow in ways that will take even a day or two off that Disney loses. Sure they will still get your money for 5 out of your 7 days, but they used to get it for the whole time. Also, if you are now deciding to spend less time with the mouse you might be more complacent with not staying in their very expensive hotel rooms or eating in their restaurants as well. So, Mickey could very well stay King, but he is going to watch his kingdom get smaller and smaller. Also, when you keep in mind that Disney has lived by this for so long that reduction could send them into panic mode.
Let's also not forget that the other player in town, Sea World, announced their own mega expansion not that long ago as well. The game is changing in Orlando where Universal and even possibly Sea World aren't just considered "those other parks". Orlando could become more of the destination, with Disney now just being just where you spend the majority of your time. If I was Disney, I would be somewhat worried and I would be trying to find ways that are going to bring people back into my parks for practically their entire vacations. You can only offer so many room discounts and free dining before people get complacent.