The whole premise is silly on many counts.
Should the Cowboys acquire more WR? Of course; they're really thin at the position. But you don't draft someone to "replace" Williams. You draft someone because they have talent and they're better than other available guys. Then you get into camp and sort out what you have and assign playing time accordingly. This idea of bringing someone in to "replace" someone else is just incredibly wrong-headed.
Also, the Cowboys threw over 100 fewer passes last year than they did the year before. Did you expect Williams' catches to increase? This offense uses Dez as the clear #1, and everything else is spread around between Witten, Williams, Beasley, and the others (Witten was the clear #2 in past years, but his targets dropped more than anyone's last year). So what's wrong with that, exactly?
On a per-target basis, Football Outsiders rated Williams as the 3rd-best receiver in the NFL last year. You can quibble with their stats, but he was extremely productive when thrown to. Yeah, let's replace the guy.