He'll be 33 in June and after 12 seasons with the Highlanders and 80-plus tests for the All Blacks he's decided that will be enough.
He couldn't convince himself he'd have the mental energy to come back to Super Rugby after this World Cup and enjoy it, and he's of the view, too that by July 2020 the next generation of All Blacks will be offering more than he can.
So he signed with Pau earlier this year to give himself the chance of a perfect exit: the opportunity to go to Japan still rated the best fullback on the planet and to potentially be part of a history-making team that wins a third, successive World Cup.
That's the way to sign off – as a winner and with everyone wanting more. It's how Daniel Carter ended his All Blacks career and of course he too then went on to play in France and now Japan with the impossible to value sense of having nothing left to prove.
And yet while Smith has unquestionably made the right career choice, he could conceivably have signed with New Zealand Rugby for another two years, maybe even another four, and the argument he had made the right choice would be just as strong.
Smith, unlike Carter, hasn't suffered years of injury trauma. He had six months off rugby in 2017 and when he came back last year, he was fitter, stronger and faster than he's ever been.
We are seeing that more obviously now as he's been a one-man show at times for the Highlanders this year.
Smith, even as he closes in on 33, does not look like a man whose best days are drawing to a close. If anything he looks a better player now than he did when he was short-listed as a World Rugby player of the year in 2013 and he could easily enough eke out another two seasons of international football if he had the desire.
Against the Hurricanes last Friday, Smith was the man who made all the difference for the home side.
His footwork was on a different level and presumably everyone lost count of how many times he was able to find a way into space when there were three defenders all within touching distance of him when he received the ball.
Several things have become apparent, or been confirmed in the last few months as a result of the way Smith has played.
The first is that while Brodie Retallick features high on most people's list of players the All Blacks can't afford to be without at the World Cup, Smith should probably sit above him.
If Smith is somehow forced out of the World Cup it will be a bigger blow than the All Blacks losing Carter during the 2011 tournament.
The second point is that while the All Blacks have been ambivalent in previous seasons about whether to pick Smith at fullback or on the wing, the evidence is stacking up to say it's imperative they commit to using him exclusively in the former role.
Through a combination of tactics the All Blacks will most likely face and the lack of consistency shown by the alternatives who have worn No 15 since 2017, Smith needs to be at fullback throughout the World Cup.
This World Cup is going to be dominated by contestable kick strategies. The box kick from halfback is the go-to play of both England and Ireland. Wales have also built most of their attacking ploys around their kicking game and South Africa haven't yet been at a World Cup where they didn't kick first run second.
For the All Blacks to win their third title, they will not only have to be immaculate in the air – catching everything, but they will also need to be able to spark something on the counter attack when the half chance to do some comes their way.
With him at fullback, incessantly kicking to the All Blacks may not make sense.
He's also equipped with that intuitive feel for when the counter attack is on and as he's shown for almost a decade, he has the ability to get past that first defender and by doing so, create all sorts of attacking possibilities.
If there is an early prediction to be made about Japan – it is that the All Blacks will be more effective at punishing teams who are a fraction off in their kicking accuracy if they pick Smith at fullback.
The thing to ponder is what if Smith is injured? What if he is ruled out of the World Cup? Will the All Blacks pose the same threat? Will they be so secure under the high ball and so well equipped to counter attack?
The answer to that is resounding no. Damian McKenzie is many things, but rock solid under the high ball is not one of them.
Jordie Barrett, like McKenzie, has good qualities but he's not in Smith's class an an international fullback.
Neither has the same composure or range of skills and vision and the All Blacks back three won't have the same balance or potency if Smith is either injured or fielded on the right wing.
And that is the final point to be made about Smith – the next generation are not ready to surpass him just yet and probably wouldn't be next year or even in 2021 either.