Just like long-serving players, two things matter in regard to persevering: form and desire.
Can he be certain in regard to the former, after so long with one team, that he's evolving and evaluating his contribution accurately? Is there a danger, he wonders, that because of his familiarity he'll slip, unconsciously, into comfortable habits that don't get the best out of himself or the team around him?
To stay on, he needs to be convinced that he's as demanding and honest as he'll need to be to ensure the team keeps developing. It's a tough job, mentally and physically demanding with long periods away from home. Only he will know whether he still has the passion for it beyond next year.
Having been involved in two successful World Cups and having played a huge role in building the most dominant team in history, maybe he'll feel - especially if the All Blacks beat the Lions in 2017 - that he's ticked all the boxes he ever wanted to tick.
Then there is the separate issue of his belief that the wider contracting model has to change - and that the All Black coach shouldn't be appointed at the end/beginning of the World Cup cycle.
That, he argued when he extended his contract at the end of 2014 through to the end of 2017, heightens the importance of the World Cup and can lead to coaches making short term decisions that don't benefit the team.
As part of that argument, he also said it's his view that a new coach can have a more obvious and bigger impact if he takes charge of the side mid-cycle and not at the beginning.
So when he says he's 50-50 - he really is.