Assuming he WILL go, and that Alan Joyce has been able to convince the board that his current strategy is the way to go, he will have no difficulty in having his recommended replacement accepted no matter who it is.
You can bet your bottom dollar that this person will be "in tune" with Alan, and would have "sold their soul" to get there.
Wirth is right to raise her profile in order to have a tilt at the top job, no matter how dumb the attempt may seem to be, but the winner will be who is best at convincing Alan that they are the best candidate.
Having said that, I don't believe Alan is going any time soon under his own steam, rather to say that he will stubbornly resist any attempt by anyone to remove him. His "plan that cannot fail" will stumble along until the demise of the whole company and he'll leave with the others carrying a bundle of cash for their efforts.