WN's Hawaii ambitions as a whole are quite competitive so I think they would be more willing to stick around for much longer than DL/SY did. Don't forget that WN still does have a pretty strong customer base here in Portland as PDX's second largest carrier. However, AS is obviously the dominant point-of-sale carrier in the Portland to Hawaii market but if WN offers fares that are low enough it would at least keep AS on their toes. When B6/DL both flew PDX-ANC, fares on that route plummeted. Perhaps a similar scenario would unfold if WN were to start PDX-HNL.
PDX-HNL on both AS and HA are currently at 1x daily (AS was at 2x daily before the pandemic) whereas SEA-HNL often has a much as 7 to 8 daily flights including 1 or 2 widebodies. That's not to say that SEA-HNL isn't out of the question but SEA-HNL would certainly be a much tougher market for them than PDX-HNL.
With just two flights a day, there's certainly room there for WN to work with if they wanted to start PDX-HNL. However, if AS responds to this by re-adding a second daily PDX-HNL flight than WN may certainly be in trouble there...
Here were the load factors for AS PDX-ATL/DTW and NK PDX-DTW in 2018:
AS PDX-ATL in 2018 - 13856 passengers, 17445 seats, 79.43% load factor
AS PDX-DTW in 2018 - 41055 passengers, 55678 seats, 73.74% load factor
NK PDX-DTW in 2018 - 4091 passengers, 10632 seats, 38.48% load factor
AS previously served ATL and DTW nonstop from PDX, but dropped PDX-ATL/DTW nonstop service prior to the COVID-19 pandemic. However, ATL and DTW are both major hubs for DL, and DL also has significantly more connecting feed on PDX-ATL/DTW than AS ever had on these two routes. DL also has a FF base in Michigan, Ohio, the Northeast, and the Southeast to support PDX-ATL/DTW nonstop service on DL.
WN would have an advantage over AS if it adds PDX-ATL nonstop service with the connecting opportunities that would be there to GSP, RDU, RIC, and Florida through ATL on WN along with the FF base that WN has in ATL and Florida.
AS could probably make the return of PDX-ATL/DTW nonstop service work if demand were at pre-pandemic levels with neither of these routes currently having nonstop competition and with NK no longer serving DTW nonstop from PDX.
STL doesn't currently have any nonstop service form PDX, but WN is also currently scheduled to resume STL-PDX nonstop service on June 6th, which is less than a month away.
While AS doesn't currently serve IAH nonstop from PDX, I have previously mentioned the possibility of AS adding PDX-IAH nonstop service in the Houston Aviation Thread with (a) UA currently being the only airline serving the Houston market nonstop from PDX, (b) WN no longer serving HOU nonstop from PDX, (c) IAH being one of the top destinations traveled to from Greater Portland that AS doesn't currently serve nonstop from PDX, and (d) IAH being the only UA hub market isn't currently served nonstop from PDX on AS.
CLT currently has nonstop service from PDX on AA but not AS, but AS doesn't currently serve CLT and AS FF's in the Pacific Northwest who need to travel to CLT are willing to fly to CLT on AA due to the AA-AS partnership.
I am unsure if AS would increase PDX-Hawaii nonstop service in response to WN adding nonstop service to Hawaii from PDX as there are a few destinations such as ATL, CLT, DTW, IAH, and STL that have nonstop service out of PDX on a US4 carrier but not AS.
Is AS likely to re-add PDX-ATL/DTW/STL nonstop service or add PDX-IAH nonstop service to better compete against the US4 in PDX?