I don't know if they've written him off already. Maybe in 2013 Cooper was better than those guys based on the eye test, but based on this preseason, Cooper was struggling mightily against a lot of guys who are not getting on the field either. There isn't much opportunity for Cooper to show that he can play in practices at this time of year; you can't hit.
I guess my POV is pretty much what other people were saying: If Cooper were illustrating in limited practice reps what made him the 7th overall selection in the 2013 NFL draft, that talent would override the continuity question. That Cooper can't get on the field right now speaks pretty loudly, IMO.
Cooper's salary and draft position are a sunk cost right now. I'm not sure why the comparison between "Cooper's totally going to earn a LG job" and "Watford is totally going to earn the RG job" is "apples and oranges" except that you (now) think that Watford has no value (although I'm pretty sure that he's been as active on game day as Cooper has) while you think that Cooper is going to seize the starting job.
I expect that Cooper will be given every opportunity to earn that job next offseason, but I see very little evidence to suggest that he'd earn it any more than Kevin Minter managed to do.
Draft position seems to matter very little to Bruce Arians once the bullets start flying.