This assumes that the NHL will be participating in the 2018 games. According to this week's issue of Sports Illustrated in the United States, the NHL has already decided not to go to Korea - they just haven't made the announcement yet.
But if the NHL should change its mind on Korea, isn't it true that Canada and Russia will likely be the same age across the board? The spark for the Canadian Gold Medal in Vancouver came from their 2005 World Juniors group - Crosby, Bergeron, Perry, Getzlaf, Weber, etc. The spark for the Gold Medal in Sochi came from the same 2005 World Jr. group. In 2018, if the NHL goes, these guys will be around 33 years old, and they'll still be outstanding players, but they'll have a little less spring and speed, and they will be trying to achieve a three-peat, which is harder to do.
Canada hasn't won a Gold Medal in six years at the WJC, which is probably an accurate gauge of where the competitive balance lies. As Brent Sutter said, back in 2005 Canada had no real competition, but now Europeans and even the US are changing the competitive balance. I predict a three-peat won't happen.