You listed two possible outcomes from putting Webb in - everything stays the same or things get better. What about the third possibility - things get worse. And no smart ass comments about how things can't get worse because we all know that they can. Look at Cambell's play when he came in for Cutler. Things can get a LOT worse. What message are you sending to the team then? "Hey guys - I really don't give a shit about you or your careers, I just want to see what happens with a different QB... maybe we'll win, maybe we won't..." That is a great way to lose your locker room.
You and I only have games to evaluate Webb and how he interacts with the other players. The coaches also have practices. Before they do anything like swapping starting QBs "just to see what happens" they will start installing Webb in with more of the starter packages. Honestly, I would be surprised if they haven't already. They can use that additional time to evaluate him and how he works with the other players without risking the season or other players futures. If they haven't seen anything that would make them comfortable with making the switch, why on earth would they do it anyway?
Finally, we need to keep in mind that we aren't out of the playoff picture yet. Yes, it is a long shot, but while we are still in the hunt (and right now we are solidly in the hunt) we need to do everything possible to win. Swapping out your starting QB just to see what happens is a sign of desperation, and NOT something you do until you are eliminated from the playoffs. Putting in Webb now would be giving up on the season.