I would argue that this does happen in SF specifically. Being a magnet for people with no leg to stand on (whether because of a lack of resources, prejudice, etc.) is San Francisco's whole thing, and has been since the 60s. They wrote songs about it!
But okay, in the case of Seattle and Vancouver, maybe the arrivals are not from the opposite side of the country, but there still is a constant influx of people from the region. There are huge native communities in both Washington state and British Columbia, and they're falling apart in about the same way as the rust belt. There's no support system in those small towns with enough resources to help the people who are worst-off, so they have to leave and go somewhere else.
But okay, in the case of Seattle and Vancouver, maybe the arrivals are not from the opposite side of the country, but there still is a constant influx of people from the region. There are huge native communities in both Washington state and British Columbia, and they're falling apart in about the same way as the rust belt. There's no support system in those small towns with enough resources to help the people who are worst-off, so they have to leave and go somewhere else.