More like people suggesting BKK and MNL, which are countries where the average cost of a J fare are something like 75-150% of per capita GDP. Sure, there will be lots of VFR traffic... in Y, at rock bottom prices (keep in mind you're also competing with some serious oversupply in markets like SEA-TPE, which has more capacity than SEA-LHR, lol, this is going to depress prices all over SE Asia).

Note that YVR-BKK is seasonal and SEA-MNL is sub-weekly.

SYD is probably better economically in terms of fuel burn to fly from SFO or LAX but AS having to start in those markets is pretty suicidal compared to SEA (where at least you're the only one flying it until DL starts it to fight you off or QF decides to expand there). IDK, maybe they could do a direct same plane service SEA-HNL-SYD?

AA execs certainly thought it was workable but the timing for starting their SEA longhaul hub was garbage (as the pandemic nuked everyone's international traffic) and UA and DL played serious hardball with AA's plans (DL adding SEA-LHR and getting VS to add SEA-LHR, UA figuring out how to eke some extra miles from their 789s to put SFO-BLR in range).

Not shockingly AA folded because that's what they do. Thing is AS probably is going to have better costs and it's not like many of their other options don't have competition- SEA-LHR (already mentioned), SEA-CDG, SEA-AMS/FRA, SEA-TYO (I am surprised they're doing NRT instead of HND, or maybe JL will swap their NRT to HND?), SEA-ICN, SEA-TPE and so on. The markets where they do NOT have competition are ones where there's reasons why nobody flies it year round (BKK, MNL) or at all (HKG).