I don't think you have become stupider. Maybe more in need of the mindless relaxation that a predictable yet entertaining film can provide. I'd think if it had been depression, but there would certainly be other signs then film watching habits, right?

I don't even bother watching about 95% percent of the films available, just because they are so predictable, but predictability isn't everything. If you've watched a Lord of the Rings or Star Wars film more than once, it wasn't because you were wondering how it would end. I think unexpected plot twists are a relatively minor tool in the filmmaker's bag, compared with great acting, great storytelling, wonderful cinematography, etc.

Could it be that when you were vegetating in front of the TV, you were slightly depressed or indulging in some kind of escapism? Then you latched on to something exciting to spend your time on that erased your need for dodging reality? And now the raw peak of that excitement has faded a bit, leaving you some time and mental energy to enjoy the subset of films that are good, even when predictable. Could that be it?