I'd love to see evidence about how ADUs affect house values. My guess is they'd increase rather than decrease due to each one having more development potential. That assumes they don't turn into dens of zombies or hobos like the nimbys appear to think. And whatever armageddon happens when someone else parks a car in front of their house.
In truth the residents are probably mostly service workers, newly-married couples, grad students, etc. (the horror!), as well as some grandmamas and grown kids as well (even worse!), plus a few airbnbs thrown in. We have 11% airbnbs so far but I bet that diminishes in percentage as more of these get built.
The economic case seems to make sense given the volume here as well as in Portland. Both have historically lacked missing middle options, being mostly houses and flats. Maybe in cities with more middle options they'd be tougher.
PS, anywhere in Seattle that allows rowhouses will
get them front and back. That's the 20% of the city that allows real density. ADUs are about the 60% that doesn't.