Is it a "good idea" to tear down the Viaduct so that Seattle's CBD can connect more directly to its waterfront? Absolutely yes.
Is replacing the Viaduct with a tunnel the best way to spend $4 billion dollars (at least) of local money in order to improve the life of the people of Seattle? No way.
Let's not confuse the desirability of a public work with making that work the highest priority in the city. A few weeks ago I mentioned to a friend that there were far higher priorities for the city than replacing the Viaduct with a tunnel. She asked "like what?" So here's an example I should have remembered to offer. I live in Seattle's "Maple Leaf" neighborhood. It sits atop one of the city's highest hills. There is a reservoir here. I don't know its exact extent but it's probably 3 acres of surface and for security is within a fenced-area of probably twice that size. There are marvelous views to the south of Mount Rainier from the location. I have heard figures of $15 million dollars to put a concrete lid on the reservoir so that the entire wonderful space could be opened to the public as a park. Such a park would add far more to my life — I can walk to it in 3 minutes — than a park in downtown Seattle. There is simply no comparison in benefit.
Such a calculus can be done all over Seattle. The long-and-short is that if we have two billion dollars to spend on improving the city, there are hundreds of smaller projects which will add far more to the ordinary, daily life than the one mega-project of a 2 mile waterfront tunnel.
Btw, as to the People's Waterfront Coalition's "tear-it-down-leave-it-down" approach, I'd like to make it clear that I am not at all against it. It would be nice if the Viaduct was gone.
But I am simply unconvinced that we can do without that corridor. And I mean "unconvinced" quite literally. I am not convinced now. I have heard the arguments and they don't bite me. But it is conceivable that the weight of evidence and informed opinion and commonsense might persuade me. I would like to hear what some sophisticated traffic engineers would come up with if given the assignment to not merely study the impact of the elimination of the viaduct but also to be proactive and to show how such would work. Obviously, such a plan must have been done or will be done in studying the tear-down-and-rebuild option.