Interesting conversation.

just to go back to the OP's question about why no OW carrier flies to SEA from HKG?

I'll take a guess...because none of them currently see anything to be gained by setting up in that market. Sounds unfair to some people who may want to fly but just there are those from YVR who choose to fly NH or DL ex-SEA, there are plenty of others from SEA and PDX going the reverse way now, and flying through YVR.

I can tell you that on my AC flights from YVR to NRT, there are a nice collection of pax from SEA and PDX and the main reason they are flying AC to NRT instead of DL is price. The difference in currencies is part of the equation, the other is the much lower actual price that AC sells these seats for on their U.S. website. AC has publicly announced a long time ago that they intend for YYZ and YVR to be international connection hubs. Given that inbound U.S. pax do not need to clear Canadian customs but go directly to U.S. immigration in Canada, this has made a huge difference for everyone - pax and airlines.

There is a possibility that the OW airlines that fly ex-YVR are meeting the needs of the Pacific Northwest. For now.

DL is spending and has spent a fortune to build up SEA. I'm not sure CX/JL/AA and partners want to have to out-spend them given the market size.

CX must have done some number-tumbling to determine that the YVR market can support a 3rd flight (even if only 4 days per week) in addition to the 2 dailies.