McCabe will/should start again. Fainga'a won't.
Slim while I agree with what you're trying to say I don't think it's going to happen. Now is the time to roll the dice for Deans and whilst he has shown great faith in McCabe, the reward just hasn't been there with the exception of the Brisbane test. Barnes is our next best bet and has the ability to take over the playmaking role if Cooper is having a shocker, something we lacked against Ireland. There is nowhere else in the side for Barnes to play other than 10 or 12, quite frankly it's too late to be changing our 10 and Barnes has test experience at 12.
The other position as you suggest is open to selection, is outside-centre. Our options there include Ashley-Cooper, Fainga'a, Horne and McCabe. Fainga'a has been a let down in the last two matches, while many still debate if he had opportunities to show his class, he simply hasn't worked hard enough to get the ball and pose a threat to the defence. Horne is on the come-back from injury, I would expect him to feature at some point as he is a dynamic, hard-running outside centre who has a shitload of pace to burn and has also been part of the Wallabies set up for a while. Ashley-Cooper is out of form, he has been shuffled around the backline but really hasn't impressed anywhere, he is the most likely of the four to get a run at 13 due to his experience there last year. McCabe quite frankly doesn't have the size to play 13 and the requirements of a 13 are quite different to those of a 12.
It will be Ashley-Cooper at 13, depending on where Deans sees Mitchell at. I think Barnes at 12 is a must, the playmaking IC that we had last year worked wonders and it's not too late to change back now, he knows what is required playing there. Would love to see Horne get a go at 13 and would have to fancy him ahead of Fainga'a and McCabe. If he's not ready, why the fuck was he picked in the squad?