First of all, even if that's only one use for it, 75 minutes from Redmond to Sea-Tac is pretty good when you don't own a car. That same commute by bus is more like 90 minutes currently.
Second, infrastructure investments pays off in ways that aren't obvious through just looking at the dollar amounts involved. But, reading this, I'm trying to imagine the sort of complaints critics of I-405 might have had in the 1960s - and how they contrast with its actual value and use today. I think we're going to look back and laugh at this whole debate in twenty years.