Well, I am not going to discount Rex just because he is Rex. He could possibly improve, or at least hold down the position while we bring along a rookie, which is my guess at what is going on. Of course, he could fail.
There was plenty of film on Jake Plummer before he became a Bronco as well. He held on to the job for a couple of years, nothing great, but solid. When he was a Cardinal, he never had a passer rating over 79.6. As a Bronco, his first three years were 91.2, 84.5, and 90.2. I usually do not put much stock into QB ratings, but the guy improved. His average interceptions dropped from 19 avg., a year to 11.75. All four seasons, he was a winning QB. He threw for over 3,000 yards twice in six seasons with the Cardinals and twice in four years with the Broncos. His average yards per pass went from 6.41 to 7.1 (not major). He never had one year of more than 60% passing with the Cardinals, he had two with the Broncos.
My point here is, Plummer did better with a change of scenery and different coaches. It is possible Grossman will do the same.
Plummer held down the fort until Cutler came along. Maybe, the Shanny's are thinking about the same thing with Grossman.