The rich folks on Mercer Island whined their way to the most expensive project in state history (at the time; UW Electrical Engineering building somehow cost more) to have the 90 go under their little paradise. They ain't dumb. And this isn't 6 lanes on 520 we're talking about - this is six lanes slicing through the Arboretum and UW on its way from the Rainier Valley to Lake City. The road itself isn't the issue - it's what the road does to the turf it cuts through. As you say yourself:

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.

Yet you seem to be arguing that an unplanned expressway is preferable to a planned expressway. Why?

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.

Right - at some point you need to figure out how to move the people without destroying the neighborhood in the process. Thus, urban planning. Thus, Waze the externalizer of neighborhood blight, thus why I hate Waze and why I think they will be sued into behaving eventually.