I don't want to pick on McCullum as Australia has done the same thing in the past decade whereby no one who is a 'star' gets the tap on the shoulder. No, they often get to dictate when their final test (more often final test series) will be and usually play accordingly - like a guy who has already checked out.
I admired Johnson last summer. He realised he was past it, probably wasn't enjoying it and despite being told he probably could go through the motions and still play, he quit. And boy didn't he look happy about it. The relief after his final test at the WACA was clearly visible and there were no tears (far me, when did grown men cricketers cry on retiring!?!?). And he looked super relaxed when spoken to during the ODIs. On the other hand, you had Clarke and Ponting who languished. At least Clarke fell on his own sword after the Ashes loss. Ponting took 2 years to realise it was time and I think he admits that now.
It must be terrible to be so talented, so good, so admired to wake up one day and realise its passed you by. The honest ones put their hands up I guess. Others have commercial and monetary considerations and need to hang on.
Mccallum shouldn't probably be playing this series, but who am I to judge. Enjoy him one last time my NZ friends because he's been a good entertainer for you and the rest of us.