AS isn't going to have to worry about this for at least the next four years. They may have won a grant with a few million bucks towards planning, but projects of this magnitude are only doable with federal funding, and the $billions to build it certainly aren't going to come under the new administration.

If and when it ever gets built, though, the airlines will probably kick and scream but eventually grow to view it as an asset to the region. Short flights, like SEA-PDX, are actually relatively expensive to operate and not always profitable, since there's competition in the market--for nonstop flights, not only do they have to compete with DL, they also have to compete with Amtrak's current service and people willing to just drive themselves, and for connecting traffic out of PDX, they have to compete with every other airline that offers connections in other cities to travelers' final destinations, so the fare they can attribute to the PDX-SEA leg is limited. All of that keeps a bit of a lid on the prices they can charge and the revenue they can recognize for the short segment.

FWIW, some European airlines have embraced partnerships with rail systems, offloading short-haul traffic to high-speed trains, which has shown to be more cost-effective than maintaining short-haul flights. It's possible U.S. airlines might follow a similar path if the infrastructure ever does become a reality.