You brought up the Disneyland comparison and Paris had the same challenges as Tokyo.
Marvel has been killing it no doubt about it and after Endgame, I couldn't see me getting any more hyped but phase 4 probably has me more excited than ever. Marvel has created a real problem for other studios. They have Kevin Feige and had Jon Favreau at the start. You have Universal and WB looking at their success and instead of copying their plan of starting small and slowly expanding, they're trying to jump straight to Avengers level of success with the Dark Universe and DCEU and have failed miserably.
WB itself has shown that it can do great when working from books when you look at Harry Potter and Game of Thrones but once the books finish and they need to keep going, they quickly run into trouble like you said.
Even having an original book with a built-in fanbase isn't a sure thing, look at how badly Disney messed up Artemis Fowl.
We're at an interesting tipping point of media consumption. With Disney+, Peacock and HBO Max, we should hopefully be seeing more creative risks being taken as budgets get slightly smaller while still being able to maintain a high level of quality.
Warner Bros biggest problem is that it's dealing with big IPs like DC and even Space Jam 2 is comes with its own expectations. Bigger expectations come with bigger budgets. Bigger budgets come with accountants and committees getting involved. Accountants and committees always result in a watered down product.
It's sad but it is what it is.