One thing that opponents of the proposed Sodo arena always seem to miss is that this is much more than a purely economic decision. It may be totally irrational, but professional sports teams are a quality of life issue for many people, as evidenced by the regional euphoria triggered by yesterday's Super Bowl blowout. Sure, there is also an economic impact (and who knows, perhaps the lost productivity will offset the extra commerce?) but the Seahawks' championship season simply made a lot of people happy, and happiness is a public good in itself, even without dollar signs attached.
Do you like feeling this good, Seattle? Then build a new arena. For building a new arena would double our chances of winning a championship—triple if we get a hockey team.* Hell, with a new arena, we would have the chance of feeling this good two or three times a year!
Sure, there are still important economic and governance issues to consider before building a new arena, but (objectively silly as they are) the intangible benefits provided by professional sports are just as important. Only the most curmudgeonly curmudgeon would argue that winning the Super Bowl isn't good for Seattle. And you've gotta play to win.
* [Yes, with both the Seahawks and the Mariners already playing in Seattle, adding both NBA and NHL teams would technically only double our chances of winning a championship. But we all know that the Mariners will never win a World Series, so honestly, why muddy the equation?]