OPINION: Ben Smith, Aaron Cruden and Israel Dagg could all succumb to the lures of Europe and we would grumble for a while and then get on with life.

At the conclusion of the British and Irish Lions series they will all have accumulated years of strong service.

And by the time the next World Cup rolls around, in Japan in 2019, Smith will be 33, Dagg will be 31 and Cruden will 30.

GETTY IMAGES Charles Piutau barges past the Leinster defence - and the referee - for Ulster last month.

For Smith and Dagg, in particular, those numbers set a challenge. Will the explosiveness still be there to resist the claims of younger men?

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Cruden's task is different.

Above him in the pecking order he has Beauden Barrett, whose trajectory appears to be taking him away from the Chiefs No 10. Below him he has Damian McKenzie, or perhaps even Richie Mo'unga, whose own trajectories appear to be bringing them nearer.

In short, the exit of the trio would be regrettable but understandable.

It did not feel that way when Charles Piutau announced his exit in 2015. He was too young, too vibrant and then suddenly gone. Even given the rising power of European club rugby it looked like a perplexing move.

It may end up costing him two World Cups and a Lions series. That is a mighty price to pay for the brilliant fullback. And as fans we have been forced to bear some of the cost - no longer do we see that step, that fearlessness in contact, that giddy sense of adventure that defined his play.

We have also been robbed of watching him mature in his role - the Blues back line could have been transformed by his class.

There are others waiting to fill the shoes of Smith and Dagg if they go.

Nehe Milner-Skudder is one prominent contender for their spots but the big mover could be Melani Nanai of the Blues. No doubt his outstanding performance for the Barbarians against the Springboks at Wembley in November piqued the interest of the All Blacks.

Nanai has something special. His footwork buys him enough space to get offloads away even when he looks to be crowded out and his play for the Barbarians showed his capacity to thrive as the standard rises.

And there will be other names who put themselves forward. Already the likes of Shaun Stevenson at the Chiefs and George Bridge at the Crusaders look like the sort of players who could excel in the fullback/wing role.

But none, yet, are in Piutau's class and he is still just 25.

​The success of the All Blacks' 2015 World Cup campaign has probably overshadowed just how well Piutau was playing prior to missing that tournament. He was outstanding during the 27-20 win against the Springboks in Johannesburg.

The reasons for Piutau's omission from the World Cup have been much debated, but none sway my opinion of the player: of New Zealanders currently playing the game, Piutau would be among the four best back-three options. Not many have Piutau's all-round game.

All has not quite been lost. Piutau has talked of returning at the conclusion of his Ulster deal. But even if that is the case he will only have one season of Super Rugby to convince the All Blacks to take him to the next World Cup.

We know from history that is a difficult pathway. Even though Piutau will only be 27 it is not the same as Jerome Kaino recharging his body in Japan and then coming back. The grind of Europe can deaden the spark.

If we are lucky we might yet see the best of Piutau in a black jersey. If we are realists we have to accept that day might never come.