Canada will go absolutely ape-shiite if this happens.
Edmonton is a very small market, but one extremely loyal to hockey. And economically the area is booming due to the oil sands. Will Seattleites walk away from their lattes to root for a team that generally finishes near the bottom of the standings? I doubt it.
The Oiler fans are maybe the best in the NHL. Their home arena atmosphere is awesome and when they came to Carolina for the 06 Stanley Cup Finals, they were so classy. They work hard, have money from the oil sands, and love their hockey. It would really hurt the NHL if they moved.
Plus, it would put more libs into the NHL fan base and remove some good conservatives.