The reason millenials are coming in droves isnt because its trendy. Its because they are at the age right now where they have had kids, a career, and have money to spend and want to either take their kids because they loved going there as children, or going because they did not get to experience it growing up and now have the disposable income to enjoy it. Remember, millenials are in their late 20s and 30s.
Yes, I'm aware what age groups millenials are in. My oldest brother is one, my other brother and I were a few years too late to be one. That's why I mentioned them. Like you said, they're the age group with money to travel and so they're the ones that decide which places are the it places to go. And like you said, it's a place people want to take their kids which is why I stated that it will continue to be a popular vacation destination.
But it is a pretty trendy place right now. Keep in mind that trendy does not equal bad. In the thread I mentioned, I used Portland, Oregon as an example. It's a great city with a lotbto do that, for awhile there, I couldn't sneeze without finding an article about how to plan a trip there, what the best places to eat where, which stores I should go to, etc etc. It was just the place to go. Now last i checked, tourism is still good there. There's still lots to do and people still visit. It's just not
the place to go anymore. Same with Disney World. It was popular before and will definitely be popular after, but right now it has the honor of being the current "must do" vacation which is giving it an even bigger boost in attendance. Eventually, something else will come along and people will flock to that as the new place you just absolutely have to go to. Again, and I can't stress this enough- this doesn't mean the parks will be empty. Crowds will go down, just not super dramatically (see Portland).
But, this is all just opinion. Maybe the current Disney Mania going on in my age group (well, a little older than my age group. My age group is poor college students) will never die down, who knows?
Edit: I'm not sure how much time you spend on websites used frequently by that age group (early twenties to thirties) like Instagram or Buzzfeed, to a lesser extent. I spend more than I care to admit. And what I see on the sites is not usually the rides, or the shows, or anything like that. Typically I see:
The latest cupcake creation. Bonus points if it's a mernaid or unicorn cupcake.
The latest pair of minnie ears.
The cliche location of Cinderella's castle.
A shopping list of cute clothes or accessories you should buy for the next trip.
Or other things like that. Aside from the occasional "20 things you NEED to know about planning a trip to the World" list, very very few articles or pictures on tgose websites are about the actual experiences of DW. I'm sure the Instagrammars and Buzzfeed writers like the rides and shows too...but if the cupcakes and accessories are what's getting them and that demographic excited, then I can't see DW holding their interest forever. Not that there's anything wrong with liking those things. But the fact that they seem way more interested in that aspect of it than anything else is what makes me think it's a trend.