What you say makes sense, and there's a lot of truth in there.
Here's where I come down on it, and it's basically a positive outlook on what you've already said.
-yes, I agree, the Potter fanbase is huge, and the Avatar fanbase is, well, almost non-existent. Even the people that liked or loved the film haven't really established themselves as a "fan community". That having been said, well, that's not a make or break--but it's a real point worth making.
-I think that Avatar will lend itself to a great environment and great attractions--which, in and of themselves, will be draws--pulling in even the people who don't really care about Avatar. For example, Harry Potter is fine by me, but I'm not really a "fan". But--what I've heard about the FJ ride makes me want to go to experience it. The same could be true for people who are lukewarm about Avatar.
-There are two more films coming out, and that will be a natural build of material to draw from, new "fans" and/or visibility to the general populace, so we needn't worry about people not knowing what Avatar is about--Jim Cameron's worldwide publicity machine will see to that.
So, yeah, Potter is a juggernaut that can't be denied, and on balance, Avatar has a bit of an uphill battle--but if the sequels are good and the attractions and theming are compelling, Disney can have a hit on its hands without there having to be a Potter-esque rabid fanbase.
About the only thing that could really compete with that would be Star Wars, and so, DOH, whatever they need to do with Lucas they should do that as well. And that point has been made here ad infinitum. We shall see.