The Seattle water shed is built on hard pan and can only realistically support 1.75 million people. Tops soil is only 1/2” deep - except in a few places, but they were filled and paved over decades ago.
After that number was achieved back in the early 80s, the whole of King county has been on borrowed time.
The much of the city is built on fill, in any case.
Yikes! That doesn’t sound good at all. What happens if they have an earthquake?