The bottom line is that not every intellectual property can be considered equals. Time has a way of rendering even the most popular franchises irrelevant. You've got once-beloved franchises like the Muppets, Loony Toons, Sesame Street, Ghostbusters, Flintstones, Scooby Doo, E.T., Goosebumps, etc. which don't garner much attention these days. You can't just point to a Disney franchise and say "well, people still like Cinderella after all these years so clearly they'll still like _______."
The best potential comparison for Harry Potter (IMO) is Star Wars. The third film was released in 1983 and no film film franchise was ever bigger. But once George Lucas swore-off making any more films, Star Wars couldn't have been less relevant in the late-80s to early-90s. No new product meant very little interest. And Harry Potter may well be headed down the same road. The question is whether HP will ever reach a point of releasing more books, more films, cartoons, action figures, etc.
Theme park attractions are hit-and-miss. Six Flags licensed the likes of Batman, Superman, Star Trek and Nickelodeon characters for rides but that didn't exactly send their attendance thru the roof.
The Spider Man and Mummy rides at US are pretty good, but that's more a testament to the quality of the attractions than its theme. I don't see people flocking to US because they like the Mummy films or Spider Man comics/films. Will the Harry Potter attractions be any good?

If they are, will it matter? Time will tell.
I'll say this: My wife is as big a Harry Potter fan as anyone. Already saw the new film on Wednesday and has read all of the books multiple times. But after our 2-day trip to US/IOA back in March, she has little desire to return even with Harry Potter Land...particularly if the signature attraction ends up being yet another simulator (which is what I've heard) at a park that already has way too many simulators.
I think the HP license could have been much better leveraged about 3-4 years ago when there were still several books to be released and the bulk of the films. Now all they really have in terms of a tie-in is two film releases. Then it's all downhill for the franchise unless Rowling changes her mind.
FWIW, I do believe Disney has a number of things in the hopper for WDW. Too many rumors now (Fantasyland, Monsters Inc ride, Star Tours 2, etc.) that it's starting to reach the point of "where there's smoke, there's fire." They have played things pretty close to the vest since Iger took over so I think it's just a question of when the announcements start coming and how extensive the additions will be.