Take a peek out the window as you cross the 520. That should have been six lanes through the arboretum. Think about traffic that "should flow like water" and then click around.

I haven't crossed 520 since the tolls came up. I dont go to that side any more unless I absolutely have to and then I just use I 90 and go in non traffic times. I dont think 6 lanes on 520 would have been a bad thing it would have prevent the 3-2 carpool lane merging shit that was constantly clogging it up on the east side. The bigger problem is that I-5 cant be expanded due to the problem with the convention center. Not sure if that was a problem back then though.

As a counter example the rich folks on mercer island have a highway running through their little paradise and I dont see them suffering too much.

The bigger problem with the Thomson express way is that it would have cut the city up too much and made too much unusable space. There would have been 2 highways running parallel to each-other less than 2 miles apart which would have kind of been a waste of space.

Some of those routes actually make a lot of sense though. The Nortwest expressway extension was my commute route when I lived in the Ballard area for example and worked north. The Bay Freeway (that instead became the mercer mess) 5th street bridge and the Vashon island connection all could have made sense (although costs could have been prohibitive).

None of this is really all that related to the original issue of pushing relatively low speed traffic down city streets. There are places in Seattle where that hasn't happened but almost anywhere South of 85th this is already a thing and density is high enough that residential streets dont really exist any more per say except in some corners of Ballard. Once you have enough people living in an area it really doesn't make sense to treat the area the same way you treat a sprawling suburb neighborhood and try to block people from traveling through.