Then again, time changes things - particularly as the novelty of the WW wears off in a few years and IoA needs more to market the park. I'm sure that there are provisions in the contract for renegotiations.
As for the vote on the JKR web site, I have no doubt that the convention-going, mugglenet crowd are likely to be more purist in their approach, but the reality for IoA is that that larger public who make up a far greater portion of the park's patrons are going to be far less concerned with that sort of canonical purity.
Eventually, the park's management may reach the point that "we're the ones paying for this park, and we need something more to draw the crowds." It doesn't take an Owl in divination to realize who will be doing most of the complaining in two years when WWoHP begins to show signs of wear if the budget for maintenance isn't what they'd hope.
At such a point, the closed-door discussions could be quite interesting, though we'll never know what goes on. Declining crowds could give IoA some bargaining leverage, particularly depending on what options the contract gives them to simply remove the Potter theme entirely and repurpose the area.
In general, it's rare for "never" to truly mean "never" - more like "not unless and until changes are negotiated". And virtually anything is open for negotiation.
As someone who is of the "convention-going, mugglenet puriest" crowd, I'd be willing to bet you some cash, that you're probably going to be wrong. JKR is known for being set in her ways when it comes to her choices for her franchise. She was completely against any official characters being in the park from day one, regardless of fan opinion and it's extremely doubtful anything will change it. Every little thing in that park had to be personally approved by her, down to taste tasting every recipe created for the butterbeer. If she didn't like it, think it was ok cannon wise, or represented her characters to the upmost of satisfaction, it didn't happen. That is not going to change. Even in regards to fan conventions, certain aspects need to be cleared through her before they are allowed to take place using her characters. Not saying it will never happen, but having been in this fandom for well over a decade now, I'd bet against it in very good faith.
Also, the "purists" are the ones that the park is going to need to survive in the long run. The conventions alone that are being held at Universal are bringing in big bucks. Infinitus last year sold out with over 10k Potter fans in attendance, Leaky Con this July is also bringing in over 10k for an entire week. The Night of A Thousand Wizards event is also going to be a huge draw this year. These conventions drew sell out crowds every year before the WWoHP exsisted and before they were held in Orlando, they are even bigger draws now that they can be held at the park. You also add in the "purist" fans who make multiple trips per year now to a park many had never visited before, and spend thousands of dollars on merch, and they are not the crowd you want to push aside for the "public at large". And the minute some man in a fat suit shows up with a fake sounding Hagrid voice, the purists will leave.
Sorry, we'll to agree to disagree. Guess I just got slightly bothered by the "convention-going, mugglenet" crowd remark. Because yes, I am one, and yes, I agree with JKR that I don't want this beautiful park to turn into some Potterfied Disney World character breakfast. I've flown down to the park 4 times since it opened, and I'm flying back tomorrow, because I love it. Yes, changes will come, but hopefully, they will stay within the same context of theme immersion Universal has already created without making it some "theme park".