All they have left is their expert thumbs, flicking up and down.
I don't think this is the only factor at work, though. People have been aware for some time that critics don't tend to operate on the same level as most media consumers, which is why every year we see at least a dozen critically-praised books and films that die in the marketplace because nobody outside New York or Hollywood wants to read/see the damn things. There's also the related issue of reviewers who basically whore themselves out to the studios, which just jacks up the level of distrust. What it boils down to, in the end, is that more people like the kind of movies Joe Bob Briggs and Harry Knowles want to see than the kind of movies Richard Corliss and Roger Ebert want to see, and by the same token they really don't care what Michiko Kakutani thinks about Danielle Steel's latest book.
People in general don't like being told what to do, and critics have always been in the business of telling people they should watch this movie but not that one, often on the basis of film-school criteria that make no sense to Joe Six-Pack and Jane Appletini. So the critics come of as snooty and snobbish, which doesn't work so well unless you're trying to sell cat food or upscale mustard. So while newspapers, magazines and TV may be giving critics just enough time to signal thumbs up or thumbs down on a movie and little else, the blogging public and their friends decided a while back to flip the critics a different finger entirely, and if that means critics are going on the endangered species list, I say it's about time.