I wonder if the writing is on the wall and the AS hub days at PDX are over...They just haven't announced it yet...
I wouldn't be that pessimistic. After all, we've gotten CUN, FLL and DEN during the pandemic. AS isn't going to just give up an entire concourse (concourse C) at their second largest hub. As flyoregon mentioned, PDX is in a very odd situation. With just a 3 hour drive to Alaska's largest hub in Seattle, adding more flights at PDX starts to make less sense for them, and Alaska's dominance at PDX prevents other airlines from making substantial moves here.
On the other hand, Alaska could definitely expand a lot more at PDX if they really wanted to. Even before the pandemic, there has consistently been lots of unused gate space in Concourse C. Mid-tier markets have been more affected by the pandemic than larger markets, despite the fact that leisure travel is shifting away from larger cities to areas with more outdoor and recreational activity, along with business traffic being down significantly.
As sadistic as this may sound, at the start of the pandemic I had thought (and kind of hoped, in a weird way) that Seattle would be seriously affected by the pandemic's negative effects on travel to a point that they really couldn't ever fully recover from since the first known U.S. COVID-19 cases originated in the Seattle area. In the long-term, I figured that the Pacific Northwest demand could start to shift over from SEA to PDX when leisure travel demand started to come back. However, both AS and DL have kept a very solid route network during the pandemic, and PDX has suffered even more as a result.
In the very slight chance that AS started to significantly scale back at PDX, another airline would certainly swoop in to take their place. I think it would be WN. DL already has two main west coast hubs. Both AA and B6 have never really shown any desire to grow at PDX. UA is the most conservative of the big 3 airlines by far, so there's no way that they would establish a new west coast hub, The ULCC airlines have come and gone (let's wait and see how G4 does), so that just leaves WN. I think PDX is the most ideal place for WN to establish their gateway to Alaska, and there's a chance that they could add PDX-Hawaii routes in the near-future as well.
However, AS isn't going anywhere anytime soon and we honestly have a lot to be thankful for. In particular, PDX has a much stronger regional route network than most mid-tier markets thanks to Horizon. Maybe we'll just have to wait until travel demand fully returns and when the PDX terminal projects are all completed in order for AS to establish a much stronger route network at PDX.