Most of what I was going to say has already been said.
I think the main thing is that a thrill park would create a non-Disney atmosphere that would further push it into a niche area. Islands of Adventure is a quality park, but it does have a certain shirtless beer-swilling element to the crowd that you don't see at Disney parks. If you eliminated the general interest attractions from the park and made it only for thrills, I think there would be a lot more of these people, and it would turn off the vast majority of potential customers...particularly those that choose to vacation in Orlando. It would be next to impossible to make it "magical".
Another excellent point is that thrill rides are very expensive. As has been said, Disney doesn't even properly maintain its existing parks, so pouring tons of money into attraction design, construction, advertising, etc. just to create a park that will repel anyone with small children (or grandparents) is not a good use of limited resources.
There will always be some attractions in any given park that don't appeal to absolutely everyone, but the idea of an
entire park dedicated to an exclusive customer base is antithetical to the concept of the Disney theme park.
I am pulling this from a website of questionable accuracy, but here's why Walt created Disneyland:
Walter Elias Disney wrote:It came about when my daughters were very young and Saturday was always daddy’s day with the two daughters. So we’d start out and try to go someplace, you know, different things, and I’d take them to the merry-go-round and did all these things – sit on a bench, you know, eating peanuts – I felt that there should be something built where parents and the children could have fun together. So that’s how Disneyland started. Well, it took many year… it was a period of maybe 15 years developing. I started with many ideas, threw them away, started all over again. And eventually it evolved into what you see today at Disneyland. But it all started out from a daddy with two daughters wondering where he could take them where he could have a little fun with them, too.
Personally, I love roller coasters, but I think they are more at home in a place like Marvel Island at IOA. I don't want to think of the Hulk and Disney in the same thought, really.
I suppose it's possible that just slapping the name "Disney" on it would keep away the shirtless beer-swillers, but that would limit potential customers even further. The net result would be that the most expensive park would have the smallest number of guests.
The night park would be a smaller endeavor that falls into a different category, like a water park, a sports-themed center, a golf course, etc. Those are definitely not for everyone, but if you don't care about them, you can happily ignore them and not feel like you're missing anything.
If there are five proper theme parks, you're going to want to see them all, and if you start off with one that has
nothing you're into, it's going to leave a very bad taste in your mouth. I have friends who visited Islands of Adventure for the first time during the Halloween nights. This limited park hours, so they missed a lot of things. They came back at night and it was both packed and not to their liking. They will always think of IOA as that really crowded place where they had a bad time.
If Disney does (or at least did) anything right, it was producing the kind of place where virtually anyone could be entertained. When they've strayed from this, they have largely failed.