I don't think they have reached the 24 flights a day limit for that Airport yet??? The news report mentioned there could be a third Airline but didn't elaborate.
Two years ago Allegiant expressed interest but backed out when they couldn't have sole control over the Terminal. Ever since United announced, there has been no other news. I hope they haven't reached capacity yet.
Paine Field is an easy 20-25 minute drive for me, while SeaTac is 1-2 hours or more depending on traffic.