WN couldn't make LAX-PDX work. I guarantee LAX-SEA would be even worse for them.
Here were the number of passengers, number of seats, and load factors for PDX-BUR/LAX/ONT on WN in 2019:
PDX-BUR - 74951 passengers, 93995 seats, 79.74% load factor
PDX-LAX - 160978 passengers, 190832 seats, 84.36% load factor
PDX-ONT - 81877 passengers, 96191 seats, 85.12% load factor
WN was likely able to make PDX-LAX/ONT nonstop service work prior to the COVID-19 pandemic with the load factors that WN had on both of these routes in 2019.
The PDEW of SEA-LAX was 2318 passengers/day in Q3 2019 (1.5x higher than PDX-LAX in Q3 2019) compared to a PDEW of 1544 passengers/day on PDX-LAX in Q3 2019.
The PDEW of SEA-LAX was 616 passengers/day in Q4 2020 (2.2x higher than PDX-LAX in Q4 2020) compared to a PDEW of 282 passengers/day on PDX-LAX in Q4 2020.
It is hard to say if LAX-SEA would perform worse than LAX-PDX for WN with LAX-SEA having stronger demand than LAX-PDX. There are also a few destinations that WN served nonstop from both PDX and SEA in 2019 such as MDW, OAK, PHX, and SMF that had stronger demand to SEA on WN than to PDX on WN in 2019.
There are a few differences between LAX-PDX and LAX-SEA such as (a) AS having more frequency on LAX-SEA than on LAX-PDX, (b) UA and B6 having nonstop service to SEA but not PDX from LAX, and (c) DL hubs on both ends of the LAX-SEA route.