Well, I guess I look at it a little differently - for me it's their career, and they should be free to pursue it overseas if they want to (as long as they don't expect to be picked for the AB's while they are gone)
If Piutau wants to run the risk of never playing for the AB's again, that's his choice. If he wants to return in time for selection for 2019 RWC, and compete for selection, all power to him ... Hosea Gear, who's just returned from doing exactly the same thing you are critising Piutau for (but is ok to be selected in your book), is trying to do exactly that, but probably won't make it, as he can't even get a starting spot for the Chiefs.
Jerome Kaino has had a stint away overseas, and was good enough to take back the All black 6 Jersey, nobody seems to worry about that too much. Luke McAlister was rushed back from overseas and into the All Blacks when they thought they needed him (precedes SBW I believe, so I'm not sure what precedent SBW set). Carl Hayman and Nick Evans were both pursued when it looked like the All Blacks lacked depth in the 10 and 3 jerseys, they both said no, but it seemed that nobody would have been upset if they'd returned at the time.
As you've rightly said, there are plenty of up and coming outside backs, although even the player that you don't rate, but would still pick (James Marshall), has had a stint overseas.
In summary, the ABs should pick the best squad from all of the available 2015 players for the RWC. If you don't pick Piutau (assuming his form is good enough), you also shouldn't pick Carter, McCaw, B.Franks, Mealamu, Nonu, C.Smith, or any other player who is unavailable in 2016 ... which is of course, ridiculous ... we'd be figuratively shooting ourselves in the foot.