I understand this is the biggest reason why people hesitate to speak about a fifth Disney park or fourth Universal park, but the counter point is there is no law in nature (or economics) which states "this is what a mature fully developed market looks like."
Many thought the movie and television industry was fully developed in the 90s. Compare that to the amount of media to today... It has multiplied by hundreds or thousands.
I saw one article basically say "Disney won't add a fifth park because no tourist can go to five parks on one vacation." That's as logical as saying "a restaurant wouldn't have more than one entree on their menu because no one can eat two in one visit." Netflix/YouTube has more media than you'll ever be able to consume, restaurants as well, why do people feel this is a strict law for theme parks? I believe theme parks and their rides will be like a menu of options to choose from for your vacation, and maybe you'll come back for more.
But I agree with the rest of your post.
(Side note, I'll step back from talking about this. I know I've posted several times on this topic already and it's diverting a bit away from the overall point of this thread.)