I didn't say I expect Jimmy to play as good as Brady in his first year, I don't expect either one of the to come in and pull a Matt Cassel, if Brady goes down, I don't see 2008 all over again, there aren't 2 potential HoF wide receivers, a dominant offensive line, and a (known) top 5 defense (we're all just hopeful right now, we don't know for sure) there to help carry the team to 11 wins. No disrespect to Cassel either, but he won 7 fewer games going from one of the hardest schedules in the NFL in 2007 to the easiest in 2008.
Brady goes down the season is basically over either way. I want the guy who is going to be on the team for the next 4 or more years getting the reps in that scenario, not the guy walking away after this season. They're both most likely going to pilot us to a relatively equal number of wins, probably between 8-10.