You didn't read my post.
A poor arrivals experience is very relevant for DL, if it cares about satisfied customers making onward connections in SEA.
If a flight arrives during peak hours, and passengers are held in the hallway for 30 minutes to 1 hour before being allowed to queue up for passport control, it's likely many that have tight connections will miss their connections and DL will have to make accommodations to rebook them.
DL, more than any other airline that serves SEA, is affected the most by the congestion of the current IAF facility, due to the operational difficulties it causes of missed connections and onward connections for its customers.
For other airlines that rely little on onward connections, there isn't as much at stake for them.
I read your post. The evidence doesn't support it.
You have a poor understanding of JV economics if you think a route like SEA-KIX (or BOS-ICN for KE) isn't more viable for the operating carrier under a JV umbrella. Profits (and losses) are shared on the back end and split by density adjusted ASMs. Thus, having more skin in the game from a capacity standpoint increases the share of distributed scope profitability, and routes that are otherwise lackluster and even loss-making can be made whole via a transfer payment with the benefit being that the JV otherwise wouldn't serve the market. These mechanics couldn't be applied to SEA-HKG because as there is no Open Skies agreement between Hong Kong and the United States, HKG is not included within the DL/KE JV scope. That being the case, yes, it's probably no coincidence that a route that could not be included in JV scope was dropped to re-start a route that is now included in JV scope.
How is SEA-KIX more
viable for DL to operate under a JV? Have you seen what has happened with DL's JVs in Europe? Is DL flying to more
secondary spokes in Europe that are covered by AMS and CDG?
Secondary markets like KIX are exactly what this JV was designed to cover. DL now gets their full share of revenue for a customer flying XXX-ICN-KIX no matter the carrier. Before that wasn't the case, and instead DL had to take the full cost of SEA-KIX to earn the associated revenue. There's less incentive to fly SEA-KIX when additional customers can be flown through ICN at a low marginal cost. For SEA-KIX to make sense, the same revenue stream as before now has to cover the additional cost of flying directly to it instead of the low cost of adding capacity over ICN. The economics are even worse for DL. There's even less incentive. That may still mean that DL will fly the route, but the long-term outlook is even worse than before.