I don't see the connection between the KE joint venture and the future of SEA.
Delta built up the SEA hub because taking on UA at SFO was not realistic and DL is already doing everything it can at LAX. ATL, DTW and MSP are very long flights to anywhere in Asia and those cities don't make any sense as a connection point for any west coast cities. They cover the SE and upper mid-west very well, but offer nothing to the west half of the country.
When fuel prices were $3/gal and lower prices were not on the horizon, Anderson bet on SEA as a lower-cost domestic alternative for long-haul flights to Asia. SEA still makes sense for most of the big Asian destinations, and the KE joint venture will not change that.
The KE joint venture will help DL with connections to all of the second-tier and third-tier Asian destinations as well as a couple of first-tier cities like SIN and BKK that aren't going to get DL nonstops in the foreseeable future. The days of the NW NRT hub were fun, but over.