We may disagree, but apparently the talent that makes the event happen (and built it up to the reputation it enjoys and is riding on) agrees with me. Walking Dead is not a horror property - say it with me. That's why A&D doesn't want to do it - they know there's literally nothing more they can do with that property that hasn't been done (and given the budget and resources they have, can't really be done all that well).
In your travels to Orlando, did you ever get a chance to visit the Crooked Spoon food truck? It was very good, so good the owners bought up an old Friendly's in Clermont and created a brick-and-mortar restaurant, where they could serve true gourmet food. The problem is, people who go to a retrofitted Friendly's in Clermont don't want deconstructed gastropub fare--they're looking for burgers. So the Spoon was forced to adjust its menu, much more focused now on (amazing) burgers* and sandwiches. I don't think the kitchen staff much appreciated it, but they did it to survive.
Creative may not like TWD--or working with AMC, which I suspect is the real issue--but at a certain point you need to accept that's what the public wants. I'm sure there are Imagineers working on a Frozen overlay who'd prefer to work on Star Wars land, but that's the job--why they call it "creative." Find a new way to make zombies interesting, make guests feel like they're in the show. Because horror property or not, a whole lot of people are watching it. (BTW, while the show itself is primarily filled with melodrama, surely the zombies count as "horror" if a near straight-up comedy like AWiL does. The beauty of HHN is that it's definition of "horror" is broad, from pure jump scares to classic film homages to comedy houses.)
In any case, it's just one house. Don't see how that can kill the event. Except maybe 2011, I can't think of an HHN year that didn't have at least one ****-the-bed bad house.
I'm sure marketing will try - I'm arguing they're going to fail. There simply isn't another property that can live up to these inflated expectations that now exist.
I agree this is a once-a-generation phenomenon. It will be a decade at least before we see a vaguely horror-related property with this much wide-spread popularity. But when it disappears, marketing will have to come up with something new, or old, like iconic icons. But in the meantime I can't begrudge Universal trying to make more money off it. It's what businesses do, and if it finances King Kong down the road, all the better.
* No, really, the 420 Burger, unfortunate name aside, is in the running for best in the area.