This is actually pretty easy.

Every single fare I buy on WN is never a throwaway ticket; I can change or cancel with no BS "we're charging you a fee because we can" fee or "no, this ticket cannot under any circumstances be changed or cancelled". This was also the case as an MVPG flying Alaska before the introduction of Saver Fares. I do a lot of flying that can go to WN (perhaps enough to make A-List). A decent amount of that AS flying I won't ever get upgraded on as a Gold- I wasn't close to getting upgraded on recent flights SEA-SJC-SEA or SEA-PHX (both are tough upgrades even for 75Ks). I can survive non-exit row coach for 2-3 hours, so the product difference between WN and AS is pretty small, and has significant advantages going to WN, especially if I'm comparing a WN Wanna Get Away fare to an AS Saver fare: I can stand by as an WN A-Lister on WGA, can't SDC on an AS Saver Fare, even as a Gold, the AS Saver Fare is use it or lose it, the WGA fare isn't.

Frankly I can take the money I save not paying for AS Main fares and use it to outright buy AS or AV miles (or whoever) and get premium cabin redemptions in programs, or start playing the CC churning game. So "but I can fly JL/CX F if I keep handing AS my wallet and tell them take whatever they want" doesn't work for me (and given that the AS premium redemption options in Europe are garbage compared to *A/OW/ST programs I don't feel I am losing too much). They devalued the program for me, so they get less of my business. The fact that I am based in Seattle means they will likely get some of my business, but on competitive routes I feel pretty free to collect SkyPesos or Rapid Rewards or whatever instead of Mileage Plan miles, and I have no incentive to retain AS status at a penalty of hundreds of dollars a year on top of my flying. Simple as that.