We did the same trip earlier this year. It was a totally full spring break train, and honestly, it would have been an unpleasant experience if we had been in Coach. By day two, the coach cars and the observation car were noticeably stinky, and people camping in the observation car was an issue. It's really unfortunate that Amtrak can't tack on a few more coach cars, especially during busy travel periods. Allowing coach passengers to spread out a bit more, and getting more seats out there to sell. The demand is clearly there.
Also, it was a bit laughable that they noted a "17 hour" drive time between LAX and Seattle. That would be the absolute fastest time you could expect, driving straight through in what would be the worst day of your life as a driver. A more realistic drive estimate would involve a stop in Ashland for some sleep, with a total travel time of 18-20 hours plus 8-10 hours for dinner, sleep, breakfast in Ashland. VERY few people can do a straight through trip from LAX-SEA. I've done LAX to PDX once, and it was horrible.