I'll play devil's advocate here, Keanu-style. You propose that movies are growing increasingly infantile and that the movie industry is increasingly infantilizing its audience. Well, superhero, sci-fi, action movies and the like, that attract large audiences and that would be the culprits of said infantilization have existed for decades. If there has been a recent change I would argue that is is that audiences are finally being taken seriously by Hollywood. Indulge me.
Avengers will likely be in the top three grossing movies worldwide this year. Granted, its an action piece design to entertain the masses and to appeal to our baser selves and inner nerds. It fan-serves a target audience, a niche that has grown in size to encompass a large part of the market thanks to careful education by the industry. But it is by no means brain-dead. I'll not argue for the artistic qualities of the movie, though I could (for those interested, the rogerebert.com review was quite excellent on that part) and I'll instead focus on the points the movie raises. It is a movie about artificial intelligence, obviously, and the dangers it entails and so forth. But the subjects of arms race, preliminary strike, is man supposed to meddle, global peacekeeping and others. All these are extremely relevant to the current world. Now, the movie is hardly a monument of philosophical thinking and offers no answers and little discussion. But it challenges its audience, be it only a tiny bit, which is more than olden superhero movies I feel (it is very possible that I am completely wrong about this). Furthermore, there is something to be said about the characters themselves, their personalities and how they interact, the beauty and the beast narrative that is developed between two of them, god complexes in real gods and kinda-gods, etc. You get the point. And this is not limited to just this one movie.
The original Avengers was also about arms races and developing dangerous weapons as precautions. Winter Soldier delved heavily on the subjects of espionage and surveillance in a computerized world, their dangers and how much of them we can allow. It was a relevant movie and will remain so for a while and that is one of the things that made it one of the best superhero movies of all time, above more superfluous efforts like Thor. And this is not limited to only superhero movies. The Hunger Games as had many things to say about the media and the controlling and manipulation of the masses though them. It has also dealt with inequality, and a specific kind of class warfare. Divergent tries to say something about identity and social construction though it fails miserably.
On the animation side of things, the current trend is often to make them joke-filled comedies and in that regard they are becoming increasingly clever. Just look at the joke per minute ratio of say The Lego Movie or Cloudy With a Chance of Meatballs. And these are not jokes designed all for a specific public. There are those that are aimed at children, sure, but there are also those that only more mature and older watchers will be able to understand. I'll not go into sci-fi because the nature of the genre has always made it a vehicle for interesting ideas but if you think it has become dumber, gaze The Edge of Tomorrow, The Matrix and Mad Max : Fury Road and tell me there is nothing to be said about them. I am even told some people find Inception interesting...
We will always need indie and auteur movies to really dig into the things that matter, to explore the depths of the human heart and to wrench knowledge out of the human soul. But blockbusters do do their parts, maybe more than ever before. They deserve some credit.
At least some of them do. Of course, most are still indigestible drivel unworthy of an afterthought, though entertaining they may be.
I'll add this : the industry is not (completely) responsible for its audiences preference for easy to consume brainless content. Always and everywhere, most people prefer the mass-appeal stuff that does not challenge them. 200 hundred years ago the most popular romances and novels among the literate were not were the literary gems we have conserved in our collective memories and that our schools would very much like for us to read once in a while. They were terrible pieces no one remembers that were easy to read and had little to say and engaged no one. And I am sure there were some awful radio shows in the 1900s that were incredibly popular. The best works will never be the most popular among the general population for its own fault and we are all, even the denizens of this fine forum (and me the first), partly, if ever so slightly, to blame for it.