[Ombudsman]

Here is ALaska Airlines' policy posted on their website -

Customers of Size

Second Seat Policy

We strive to best serve our customers' unique needs by providing a comfortable and safe flight for all passengers.
Alaska Airlines requires the purchase of an additional seat for any customer who cannot comfortably fit within one seat with the armrests in the down position. We are unable to guarantee travel for those who require additional seat space on a given flight unless a second seat is purchased in advance.
If all flight segments between origin and destination depart with an empty seat, customers who have purchased a second seat can receive a refund of the second seat charge for that portion of their trip.

Was the person sitting in 28A able to do so with the armerst between you and her completely down? If not then Alaska will most likely not do anything about it especially since you found another vacant seat on the plane. Had you been forced to sit there for the full duration of the flight then you would definitely be due some compensation but fortunately you were able to move.