This is true for most mass media, but interestingly it doesn't seem to be true for television. I don't watch much of it, but what I do watch seems to be vastly more interesting and less intellectually shallow (though I still wouldn't call it "deep") than the Eighties/early Nineties shows I grew up on -- and the expressions "boob tube" and "idiot box" are much older than that, so I suspect this process has been going on for a while.

Reality shows are an admitted exception, but I'm pretty sure those are just a flash in the pan, popularity-wise.