McMahon was a first team player when he left. He turned his back on the Wallabies for cash because he knew his body wouldn't hold up. Hate this narrative that it was somehow Cheika's fault. If you are going to bash the bloke at least correctly identify why he is failing.
He is a successful coach at both provincial and test levels. His style of management is just not suited to long term stability. He is a man motivator. He has very little affinity for strategic or tactical thinking. When he took over, he had a fairly highly skilled, knowledgeable rugby team that was massively under-performing. He took them, identified a few shortcomings, patched them with European based players and gave them a kick up the arse all the way to the RWC final (see Waratahs and Potgeiter, etc).
Post RWC his job was to rebuild a team with a lot of aging talent gone or going. And whilst he has blooded a very large number of new players he has largely failed, because his skills are no longer suitable to his role. His newly apparent shortcomings could have been compensated for byassistant coaches, and perhaps a selection panel. The only useful assistant we had left to become a head coach of a rival team. Larkham and Grey, despite clearly demonstrating their inability at lower levels, still got hired for the job largely because they are Australian. Our very own affirmative action program. So now the issue is not purely on Cheika but also a systemic one. The wrong people hired for the wrong job across the board.
Now, if Australian rugby wasn't on it's knees and we were still churning out naturally talented players, the provinces were performing and people were watching and playing then we would probably still be seeing some half decent results. But we are reaping what was sewn decades ago.
So you can bitch and moan about this selection or that but the issues are fundamental both with Cheika and Australian Rugby.
Edit: If i had to plan succession i would get in Deans or White on a one year contract, with whatever assistants they wanted. Post RWC i would hire someone like Lancaster.