I have to strongly disagree re/ the depth chart--Sanders is an absolute lock to be the WR2 if he stays. He is (deservedly) higher on the depth chart than Cotchery or Burress (who didn't dress some days last year...Burress is purely a depth signing). If he stays, he's starting, pure and simple.
As far as rookies go, I guess it would depend on the rookie, but barring rare exceptions it's more typical for rookie WRs to be eased in during their rookie seasons. *Maybe* I could see a Tavon Austin coming in and stealing a few snaps, but much more likely would be a fast-developing rookie WR getting third down reps at a different position than Sanders. I don't see any Larry Fitzgeralds being available at #17, so I seriously doubt someone would outright take his WR2 spot.
Re/ the "giving up on him" and "turning on him" comments: I'm a Steelers fan who values Sanders--he is a starting-caliber WR--and like most Steelers fans would very much like to keep him in a perfect world. But I also see that his value currently is as much in "projection" as production; that he has had injury issues in the past; that are tight against the cap; and that if we let him go we'd be guaranteed 3rd round compensation for a guy that has a very good chance of simply walking after this year. That is a calculated judgement, not "turning" on a player. To paint the Steelers fan base or media as somehow ungrateful or myopic regarding his potential departure is to utterly ignore the (critical) surrounding context, which obviously factors significantly into our opinions.