Disney World is supply constrained because they have chosen to be such. Much like a monopoly, Disney will set supply to meet the target profit in the face of demand. Disney still has plenty of real estate with which to build upon, including in the existing parks. They are supply constrained because for the last 10+ years, their strategy has not been to increase supply (i.e., park capacity), but rather to "better utilize" existing capacity. FP+ was all about shifting demand to lesser demand rides. The next trick up their sleeve is to double sell the parks, with a standard ticket and an after hours ticket. Now they can cater to twice the guests with the same amount of capacity (in their minds). Disney's capacity today is, I believe, probably lower than it was 10 years. I've not done the math, but seeing as there have been no major additions, and many losses or replacements with diminished capacity to what was there before, I'm fairly confident that's accurate.
Of course, we would all prefer that Disney build more capacity so they could increase the quantity of sales. Instead, they are focused on (1) the "quality" of the guest they want to sell their limited capacity to and (2) reducing the hours a ticket is good for so that they can then also sell an after hours ticket. And then, they don't need to ask the board for investment money, they'll continue to bleed the parks for every once they can get, and sadly, continue to reap the value from the hard-plowed investments of those that came before them.