It may be increasing due to people not reading the paper. Have you read a "news" paper lately? The writing is terrible. Why read poorly-composed and poorly-edited articles slapped together under a short deadline when you can pick up a good book that was crafted by the author over a longer period?
I get a couple newspapers, the writing in the WSJ seems pretty good on the whole, though I think I'll look at this morning's copy with a more critical eye...OTOH, the Miami Herald can have some pretty sketchy writing. I suppose, that if literacy is decreasing, it only follows that the writing skills of newspaper journalists would lessen as well, in step with those of society in general. In any case, you are probably right, where the reading of newspapers once sharpened our literary skills, the opposite is now true.