Regardless of all that it's actually probably fairly easy for someone to get away without a valid plate for a long time around Seattle. Lots of traffic, not a ton of cops and they're often busy dealing with accidents or other significant incidents. The stickers are only on the back so you have to have a cop see the rear plate and notice the sticker, which practically requires being right behind you. I'm also sure that expired registrations are low priority enforcements for them.