I like Avatar. I own the Blu-Ray and think it's a pretty good movie overall. That said, it's undeniably far less popular than it once was (though, its popularity really had nowhere to go but down). So maybe that continues and its time being significant in our culture is over. Maybe.
I wouldn't take that bet, though (and neither did Disney). Betting against James Cameron has historically been foolish. Remember when everyone thought Titanic was a massive mistake and financial catastrophe before it was released? The man has some kind of innate ability to capture the attention of the largest and broadest of audiences. He's been working (for years now) with a bunch of smart writers on Avatar scripts (and perhaps Disney had access to them). It's a reasonable bet that the coming Avatar movies will be both popular and good.
Cameron also has a (smaller) track record for making great theme park additions. Terminator is definitely dated now, but it's still a pretty fun show.
The point is, Disney is betting on Cameron to continue to do what he's been doing for almost his entire career. It's a safe bet, and the fact that Avatar isn't as popular as it used to be doesn't really mean much.
Also, I can imagine that even if someone hasn't seen Avatar, or doesn't like it, that this land is still appealing. It just looks neat. Are you not going to enjoy a simulator where you ride a dragon over a gorgeously rendered fantasy landscape? I mean, that'd sound great even if I hated the movie!