It has come to my attention that a group called 'Park My Viaduct' is attempting to make the case for saving the Alaskan Way Viaduct and turning it into a park similar to the High Line in NYC. I think Scott Bonjukian summed up a lot of good criticism of this plan in his post 'The Alaskan Way Viaduct Must Come Down'. My additional thoughts are below.
The High Line in NYC is a great public space, but let's not delude ourselves into thinking that the old Alaskan Way Viaduct is presenting a similar opportunity to Seattle. The conditions between the High Line and the Viaduct are vastly different. And although the High Line in NYC is a great success, having a vibrant street life and waterfront connection should be a much higher priority then bending over backwards to retrofit a structurally deficient concrete eyesore.
We have a great opportunity to relink downtown Seattle with its waterfront. Separating them by constructing the viaduct was a great mistake of the last century; let's restore sightlines to reintegrate the waterfront with downtown instead of preserving this barrier. (Side note- have you ever stood in Pioneer Square or Occidental Park and wondered how far you are from the waterfront? Although you're only physically two blocks away it's hard to conceptualize this distance due to the blocked sightlines.)
The condition of the Viaduct speaks for itself: it's falling down. What point is there to spend considerable amounts of money retrofitting and restoring a structure of little historical merit?! Is a monument to Robert Moses and 1950's era automobile culture really something we want to construct and celebrate? The High Line has great historical merit as an industrial rail artery of the 20th-century and serves to tell the history of declining industrialism in our urban centers. The Viaduct does not have this story. Spending money on preserving this would be a waste. Particularly when so many other great opportunities are at stake. (Although I believe your estimates of $20 million/acre for an I-5 LID are somewhat low, I would support this project any day over the viaduct)
Additionally, the High Line was the only right of way opportunity to create a park. The rail trestle passes over numerous private lots (even through buildings), so the park could not have been established if it were not for the above-ground structure. In Seattle the right of way is preserved down to the ground as this space was reserved as a street and surface parking. Because of this, there is a significant amount of surface-level land that has the opportunity to be redeveloped if we are careful. Any Viaduct park would have limited access whereas a surface level park would create a synergistic bond between the waterfront and downtown neighborhoods.
Better projects to aspire to are San Francisco's Embarcadero/Waterfront and Boston's Rose Kennedy Greenway. Both projects are on/near waterfronts both projects involved the removal of 1950's era viaducts, and both projects have been extremely successful.