"We may not have satellite measurements earlier than 1979, but we do know that the Northwest Passage has not been sufficiently ice-free to allow easy ship travel anytime in the last couple of hundred years."

But presently we don't have easy ship travel thru Northwest passage. Unless what you calling easy travel, means using a ice breaker ship.

From article cited by McFly:
Many Arctic researchers warned that it was still far too soon to start sending container ships over the top of the world. “Natural variations could turn around and counteract the greenhouse-gas-forced change, perhaps stabilizing the ice for a bit,” said Marika Holland, of the National Center for Atmospheric Research in Boulder, Colo.

But, she added, that will not last. “Eventually the natural variations would again reinforce the human-driven change, perhaps leading to even more rapid retreat,” Dr. Holland said. “So I wouldn’t sign any shipping contracts for the next 5 to 10 years, but maybe the next 20 to 30.”

If there was a time period in the summer in which easy travel was possible thru NW passage, then container ship will be sent to using this route because it can be a much shorter route.

With today's technology- satellite information, weather forecasting and much faster ships this "edge" should allow a greater possibility to successfully travel thru NW passage without using a ice breaker. And it seems to me that if short sections of the passage and/or only during some of the time "in the season" a ice breaker is needed, it seems to me the business model could be worked out that made it economical to use the NW passage.