There comes a point where the system (Seattle) can't support additional traffic (immigrants) without expanding. Expanding pushes out people lower on the ladder, which is often people you could consider locals, which often changes the underlying chemistry and vibe of the city.

I grew up in Oregon and most of my friends and family have a general disinterest in people moving to the state, especially from California. A neighboring town, often regarded as one of the better towns in the area, has seen drastic immigration (mostly from California) in the past 50 years. On the surface, you might not see much damage, and in fact you might see only improvements. But when you look at the side effects, such as multi-generational families getting kicked out due to excessively high taxes brought by wealthy Californians, and general road traffic increasing in speed beyond what locals want (Oregon doesn't have any speed limits above 65mph, AFAICR), you see another story.

Immigration is great when it works out for you, and not so great when it doesn't.