I think HP has at least enough staying power for a decade, maybe more, on the basis of what others have already suggested... it's well known by current generations.
The obvious (and relevant) comparison I'd offer would be Star Wars, which had a ride in DHS back in the early 90s, that was popular even more than a decade after the first trilogy, and nearly a decade before the next trilogy started. And it has remained a popular piece of IP for at least some portion of the population (else how does one explain the enthusiasm for Star Wars weekends?) I have no reason to doubt HP can hold a similar level of popularity, maybe even more so, since it appeals to a wider audience.
The one point on which I'll have to disagree with a PP, however, is that Disney made a mistake by not winning this bid, and can't compete. I don't see Disney dying any time soon as a result of not having Potter. It's been popular for Universal, but even with that said, to conclude it was a "mistake" for Disney to not get it is frankly a bit short-sighted:
The value of the investment is based, at least in part, not just on the appeal of the IP, but what it might represent in terms of incremental growth for the company. In the case of Universal, one might argue that prior to getting HP, their popularity was pretty stagnant, and customer loyalty was not exactly their forte. Adding HP represents a huge influx of capital from guests who visit Orlando without hitting Universal, and possibly a handful of visitors who come to Orlando solely to see HP.
Had Disney purchased HP, their incremental growth would have been almost exclusively from those guests making a special trip to Orlando to see Potter. Sure, those already visiting would have loved seeing HP at Disney, but since the overwhelming portion of vacation-goers to the Orlando area are already including a visit to Disney (if not an outright Disney vacation), HP would not really represent much new growth.
Consider also that Disney is already well known through much of the world, particularly Western Europe and South America, and gets a pretty good influx of guests from those locations (even just once-in-a-lifetime visitors). I am not under the impression that Universal is nearly as well known and legendary outside of the US, and can't say I've ever heard of someone taking a trip to the states to visit Universal.
One last point to consider is that the failure of Disney to reach an agreement with Rowling may also have been related to the specifics of her involvement in the project. It's already pretty well established that she had considerable control over how certain aspects of Wizarding World are designed, and even some of the operational details (no sale of Coke, IIRC?). It's very possible that Disney simply was not willing to acquiesce to her demands on that front. And who could blame them if that were the case? Disney's strongest selling point has always been in design, immersion, details, and story; they create an experience that many find more enjoyable even in spite of rides themselves that are, on paper, inferior, to some others (thrill rides come to mind).
So, basically, I don't think HP will run it's course for many years still, and (despite not having seen it myself yet) think it's a great addition to Universal. But that doesn't mean necessarily that Disney lost out or somehow made a mistake.
Hogwarts and the imminent Diagon Alley are big reasons for their love affair with Universal.
