If you only go to the south end for Ikea, Southcenter, and Seatac, how often do you really need to do those things? It's not like it becomes impossible to drive there, it just takes a little longer. Frankly, traffic in Seattle just isn't that bad compared to other major cities. I just checked the traffic from Ballard to Ikea, and it's 45 minutes on 99... I-5 isn't even the ideal route right now.

Of course I'm playing devil's advocate a little bit here, but you also have to weigh this against just how much additional real estate would come available if I-5 was gone. I don't think people's need for driving convenience actually stacks up that strongly against all the other positive considerations.