The problem with the LEGO movie is that it is a comedy about itself. It is a parody of LEGO, people's image of LEGO and some other movies - no ORIGINAL stuff at all. A great topic for a movie, indeed. But a pretty lame topic for LEGO sets. How long will people want to build sets that spoof the sets they have bought over the last decades? Benny's spaceship parody of a classic spaceship? The idea is new now, but will pretty soon get old.
I know from experience how merchandise from TV series performs while the series is running. You missed the 1st series of McFarlane figures from The Walking Dead? Hard luck - some of the figures sell for $250 now. BUT: when the series is over, interest will vanish quickly. The same happened to the LOST figures. Prices went up, up, up while the series was on TV. And when it was over, you sat on all the stuff you hadn't sold early enough while the hype was still on.
So, when the movie's hype is over, no one will care about the 1st wave of sets ever again. If there is a second movie people will rather buy those new sets. The only scenario I see in which the Cow and other weird stuff might end up STAYING popular, would be if the LEGO movies become really iconic, legendary stuff - like Star Wars, Lord of the Rings, Harry Potter, Indiana Jones. But they just won't. Because the LEGO movies won't ever have an epic story, no epic universe around them, as I said before, they are only a parody. Or am I just not aware of the big fanbase of the "Superhero Movie" or "Scary Movie". I don't think so,