I'd be shocked if people would spend 15,000 miles to fly First in UA up and down the West Coast as opposed to 5,000 on AS.

And unless you are a top-tier (1K) on UA, miles are the only way you will sit in First as there are a plethora of 1Ks cash in their 500-mile upgrade certificates on those routes or folks like me (mid-tier) who use SFO/LAX as a connection going East and used 15K miles to upgrade the whole itinerary.

UA Shuttle used to offer free same-day First Class upgrades to 1Ks, but that perk died with Shuttle.

So AS could eliminate First up and down the West Coast and suffer little, if any, retribution for folks who fly those routes. They could keep First to and from AS, across the contintent, and down to MEX to remain competitive against UA/AA.