Giteau was a failure as a 10 - hence Cooper was bought in at 19 or 20 and with no ability or desire to tackle, despite this the wallabies are better with Cooper.
JOC (James O'Connor) is a better like for like replacement for Cooper if he gets busted, the backline stays the same and Mitchell comes in if his fit.
Giteau was a failure outside Cooper, McCabe is the perfect foil for Cooper as he does everything that Cooper doesnt and can run off him as well.
Faainga can fill in for McCabe if required and again the structure and skill set of the backline remains in tact.