So why not give Greenacre the job right away? What does he need to prove? They know him since years, is he ready to step up or not? Does he really need to do another 'internship'?
Maybe it is not Greenie that needs to "prove" something. He has the badges. Maybe he sees his current job as "doable" but has reservations about what the first team coach of the Nix can do or cannot do to succeed.
Also, why assume that Greenie sees himself as a first team ALM coach? He may be very happy to help with the rest of the season run, but may actually prefer to work long term with younger players where he can refine their skills and while they are still "trainable".
Sure, we all like ambition and self-belief, but his ambition may be related to younger player development, not to guiding the first team, what with the stress and limitations it entails.