DaCostaBR: But isn't the "Brainy Brunette" trope older than dirt though?
It's not the trope, it's the actual actor. She's insanely popular. Check out her Twitter. Etc.
I think I am understanding better what you are getting at, though. But I would approach it from a different angle.
Several years ago, I was at a talk, and the speaker used a scene from The Godfather as an enlightening example. As soon as he finished his thought, he paused, and looked around the room at the ~300 audience members. He asked, "Show of hands - who here has not seen The Godfather?" More than 50% of the audience raised their hands.
From the beginning of recorded media through sometime in the mid-80s (maybe a little later than that), the cultural experience in America was pretty monolithic. Everybody saw The Godfather. And there weren't so many films out at the theater to crowd it out. E.T. was number one at the box office for more weeks than any blockbuster is now -- including Avatar. Because there were less movies, less releases. Everybody saw E.T.
Now - and mainly due to the internet, there is so much entertainment being produced, no one could possibly consume all of it - or even all of the most popular stuff.
There is no more monolithic experience of culture. (I think culture is too big a term to use just for the arts, but that's another discussion.) So, as mind-bogglingly popular as NCIS is, it is remarkably easy to live your life without ever bumping into it. Same with Avatar - which I never saw.
It is also much easier to get immersed into online bubbles that exist around your paricular tastes. In 1972, you saw The Godfather, because there wasn't much else to do. Over the six months it was at the theater, you were probably going to end up going to see it at least once. Now, I go to my computer daily to immerse myself in the arts I am interested in - RPGs, hurdy gurdy music, fairy tales, etc. And, to my amazement, find I have missed out on more than 10 Marvel comic movies. I mean, I saw the first X-Men movie and then nothing. I take a flight on a plane that is showing the latest X-Men movie, and I realize I have missed nearly a dozen relevant movies in between! Astounding!
So, I agree with you that it is quite amazing that something like Avatar can be so popular and then so quickly become irrelevant. But perhaps we disagree on why that might seem to be true.
Cheers!