As for your list of carriers and your assessments you are pretty spot on, except that I can tell you that EK very much wanted to operate the Whale on the DXB/SEA route but as noted, the Port wasn't going to put the money into airport infrastructure for one flight a day.
After thinking it over I've come to the following conclusions for SEA A380 viability:
Asiana-Not even SFO gets the A380, no chance a smaller economy and leisure-heavy destination (relative to the SF Bay area) will get it. Also SF has a much, much bigger Korean population (the Puget Sound is like 70% white or some crazy high number). Asiana already has 92%+ loads on ICN-SEA using the A350, and before that the 77L. OZ only has four A380s and has to use them wisely. Lest I forget, not too long ago OZ was sending A330s to SEA
SQ-Has no A380 routes in the Americas besides JFK-FRA-SIN, and at least that's justified because, I mean, it's New York/Frankfurt/Singapore, the three Alpha+++ financial capitals of the world. Their other U.S. routes use the A350-900 or -900ULR. I also think SQ kind of shot themselves in the foot when they went all-in on SFO/LAX. Anyway, besides O&D tech demand to Singapore (e.g. Grab), isn't the SIN-SEA flight also designed to carry tech traffic to BLR/HYD (since the U.S. carriers have given up on India)? That would make sense for a 3-4 weekly service
EY/QR (which gets suggested by many A.netters)-Etihad is in a financial death spiral and Qatar Airways has been crippled by the diplomatic crisis with the GCC countries and their cozying up to Iran. The explosive growth of Turkish Airlines has not been kind to QR either. In fact, Etihad pulled out of SFO and reduced LAX to 4 weekly. SEA is out of the question for either airline. Etihad may fold by 2025 or later. As for QR, there would have to be a new President in the White House, and Qatar will have had to have made peace with the GCC, before QR can make SEA work. And even then, a 77L would be more likely than an A380. SEA already has a Oneworld TATL partner in BA.
Lufthansa-None of their flights to SEA include first class service (333 in winter, 744 in summer; MUC-SEA will use an A350, and all of the A350s are 3-class) buuuut...with their Indian expansion out of Munich and pre-existing connections out of Frankfurt to the Middle East, India and Africa, I could see an upgrade to more premium-heavy aircraft. 343, 74H, the future 777-9X are all possible
BA-I think BA won't stick with 744/772 to SEA forever, it's just that I don't think the A380 is in the cards. Currently the schedule to SEA looks like this:
BA48/49 (evening flight): daily 772 in winter, daily 744 in summer (mix of Hi-J/Super Hi-J), 14 weekly
BA52/53 (afternoon flight): 3 weekly 772 in winter, daily 772 in winter, 10 weekly
* In summer, sometimes a 77L or 77W will be mixed in with either flight, depending on what's available.
There's a lot of cruise ship traffic and tech industry traffic. BA is the only European carrier to fly to Hyderabad, and has competition from Emirates and Lufthansa to Bangalore. But to be honest, I really don't know if Alaska cruise ship traffic is premium-heavy, or premium-light. Anyway, BA will probably want to free up a T5 slot in the future, assuming the Heathrow third runway project gets canned. I could see a daily 777-9X replacing 10x 772/daily 744+772. But that'll have to wait for BA's 777X orders to arrive
The bottom line is, Seattle might be big, but it's not that
big, especially compared to San Francisco or Los Angeles. Outside of Bellevue/Overlake/SLU tech demand, a lot of the Puget Sound air traffic is VFR or leisure, reflecting the region's blue-collar economic engines of aerospace/shipping/logging/military. YVR is similar in that regard.
I also think that the dominant carriers at SEA kind of work against it in terms of A380 potential. The dominant carriers are Alaska (non-aligned) and Delta (Skyteam). The only Skyteam carriers that fly to SEA and have A380s in their fleet are KE (too premium-heavy) and AF. I have absolutely no idea what the loads are on AF's CDG-SEA route, or which kind of markets AF wants to capture, but I do know that AF recently upgraded BLR from a 333 to 77W(!). At the same time, the DL/AF/KL joint venture allows for a bit of load balancing; DL has its own SEA-CDG flight, and AF uses the 332 as a result.
By the way, why would Emirates prefer an A380, which has less cargo capacity than a 77L/77W? Don't the DXB-SEA flights ferry 777 maintenance parts from Boeing Everett to the maintenance base at DXB?
Continuing on the subject of EK, I am curious as to the rough proportion of origin passengers. How many are flying Seattle-Gulf, and how many are flying Seattle-India, Seattle-Pakistan/Sri Lanka, how many are doing PDX/SLC/DEN-Gulf, or PDX/SLC/DEN-India, etc.? And how many pax are doing YVR/YYJ/YYC/YEG-India? (the WS/EK codeshare can also feed BC/Alberta pax onto EK's nonstop flights from YYZ or LAX, and I think SFO too)