I was an EPCOT-touring kid of the 80's and I remember having a grand old time at that park. For me, the thrill was just being on vacation in DISNEY WORLD (!). I certainly wasn't saying, "Meh, it could use more princesses". I don't know if today's kids have more in their everyday lives and are therefore more jaded, or if their parents (and in turn, Disney) just don't give them enough credit and assume kids couldn't possibly like anything that doesn't have a pie-eyed character slapped on it.
For the record, and this is a point Disney seems to be overlooking in the last several years, not every kid is into princesses. I definitely wasn't. My thing was anything animal related (I died a little on the inside the first time I saw Nemo had become the focus at The Seas). For another kid it might be space... or trains... or robots... or...
Disney could base an attraction on just about anything if they put enough effort and creativity into it. I'm convinced they could create a ride on a dump truck that explores waste removal services and still have a hit on their hands so long as it's fun and captures the imagination. But I guess it's easier to keep using characters to re tell the same old stories everyone already has in their DVD players.