I really don't think its possible to answer this question yet. You'll need to see how the sequels invade the cultural landscape in a couple years.
I would definitely agree on this point. The sequels are one big question mark at this point with a lot of variables to consider. the 3D fad has been fading, and the novelty of some of that shiny new tech has worn off. So then you are left with the story... I think that first sequel especially needs a much stronger storyline than Avatar did. And Cameron isn't exactly known for his prose. I don't necessarily think Disney would regret trying to do Avatar even if the sequels don't do as well (I don't expect them to flop), but it will probably make them at least a bit nervous.
I see Avatar as the CBS of blockbuster movies. It may not have as much geek cred as Star Wars, LOTR, or Potter, but it had wider appeal amongst the middle aged masses than many give it credit for. So, while it may not have the internet support, it does have mainstream familiarity amongst the casual consumer. Those people don't necessarily buy wands, or custom action figures, but they do bring their kids and grand kids to WDW. And something like Avatar might be enough of a draw for them to have them spend a day at AK during their trip.
Not sure I've seen the wider appeal you speak of with the middle aged masses. Curious as to why you think that? Avatar could certainly be a draw to AK if done right, no doubt. But to be really successful, the merchandise has to move... you need those people to buy the wands or the action figures or the princess stuff. Its the reason Carsland exists. And it is a core part of Disney's business model. Getting people in the door is just the first step, but it's only the first necessary to get to profit. The other steps need to happen. I'm not sure Avatar as a property can do that. Though I hope it's successful. I don't actually want the land to fail, and I think it has some potential. If we're going to end up with it, might as well hope for the best.
And as many on here like to say, for better or worse, Disney is targeting that demographic through meet and greets and rides focused at the younger set. Avatar reads to me as something targeted specifically at their parents, not necessarily the same people Uni is going after with Potter, Transformers and the like.
I would argue Uni is going for that same set of "parents" just as much as they want honestly any age group. Disney does have the edge in kids, while Uni may have the edge in teens, but each company wants it all. And Potter especially, though even Transformers appeals to that parent set. A generation grew up with Potter, but you don't get that mass success without a much broader age base that loves the product. It's a "4 quadrant" series, to use some box office lingo. Transformers may be a little less appealing to the parent set... but those films for example, weren't just marketed at teenage boys. They were marketed at older men especially as well. The "nostalgia factor" was used the same way Disney uses it for their products.
For Avatar, it's again, a very hard property to read. A lot of people saw it. A lot of people liked the tech. But the general consensus on the story was kind of a take it or leave it deal. The parents angle is hard for me again to understand, because I really just haven't seen that. Anecdotally, my 50+ parents have both seen all of HP, saw Avatar, and will probably see the sequels. But if you asked where they'd rather "visit," it's off to Hogwarts we go. Obviously it's different for different people. My point is, I think the demographic of say an HP, and Avatar, are more similar than you think.