All Seattle cares about is the Seahawks. I think Seattle has too many people with very limited free time pouring 60-something hours a week into soul-sucking IT/software jobs for there to be much of a fanbase for teams playing more than once per week. The Mariners are the only other big-four team up there right now and, despite having a baller stadium, they can't even pull enough scraps to stay out of the bottom third in MLB attendance. People act like the Sonics were Seattle's Cleveland Browns when they moved but in reality they were one of the most forgettable, whatever franchises in the NBA and had a following to match. A Seattle NHL team would probably do better than Arizona/Florida/Vegas but not by a whole lot.
And that's if they put the arena downtown. If the arena goes up in Tukwila or whatever Glendale North they were throwing around last year before the bidding deadline, forget it.