What is a "lowest common denominator demographic," anyway? How do I know it when I see someone who fits that description?
In medieval times, storefronts used to avoid words and use pictures to describe what they were selling because the public was so illiterate. That's kind of where we are now with movies because they have to not only be mainstream for US audiences--they have to be able to resonate for international ones as well.
Therefore everything waters itself down to "good guys" and "bad guys", so the dialogue (which is going to be subtitled or dubbed) won't matter and any localized cultural references will be few and far between.
This is how Hollywood today operates.
And you also have to think about the thought process that goes into deciding whether or not to plunk your money down for a ticket. Why is it that these CG kids movies are so popular, for instance? Over the weekend I will probably offer to take my daughter to see The Croods. I've already gotten wind of lackluster reviews of it. Why would I do that? Because I need to take her out and do something special since it's been a long time. The money I plunk down is not a validation of the movie. It's because the movies are a sociological phenomenon. We've got a 3D flat-screen at home now. We don't specifically need to go to the movies at all. But we go to "go to the movies". And that's what LCD is. The minimum entertainment experience people will want when they "go to the movies" is spectacle. And that's what Transformers, ST, Marvel comic book movies, that's what they all do.
Back before home-video and all the other competition for people's attention arrived, movies were a more dominant medium that covered a more complete spectrum of topics. And that includes sci-fi. Compare Prometheus to Alien, for instance. Alien was a rather intimate horror movie compared to the epicness of Prometheus. The difference in tone reflects the different role that movies play in people's entertainment now vs. then. Audiences demand to be bludgeoned by shock and awe. If they want something more cerebral they will watch an HBO drama.