Should we play..."If Kubiak can't..no one can?"
If Kubiak can't - he can't. I wouldn't think any less of him especially if, according to your scenario, he benched Carr.
It could be any player on any given Sunday, you know - everyone is just polarized over Carr right now. Next week, next month, next year it could be AJ or Williams or any other player.
If a coach has the cojones to bench a franchise player/starter if he isn't performing, I say hats off to him. That is his part of his job, after all.
Just because Kubiak thinks/thought that Carr could be "saved" doesn't make him a bad coach for being wrong, if it turns out that Carr can't be restored to serious pro-QB form. Most people make decisions based on past experiences and Kubiak's past experiences in training QBs is excellent.
I trust Kubiak and I do believe he will manage a miracle of sorts where Carr is concerned. If not? Oh, well - I see no reason to hang Kubiak from the nearest goal if he can't pull this off.
Let's see what happens against the Eagles - I think that will be a pivotal game in foreseeing Carr's future. Hopefully we will see at least a flash or two of brilliance.