No, DL does not fly ATL-ICN but the point is that NRT is a larger destination, DL is the hometown carrier, and DL and KE provide the only passenger service from ATL to East Asia. It should be obvious that KE is significantly undercutting DL in order to fill its planes.
If it isn't obvious, I can tell you the names of a number of friends who travel from ATL to Asia and who choose KE because their fares are lower - and in many cases a lot lower.
It isn't just about ATL though. DL dialed back the relationship with KE because the relationship was very much imbalanced in favor of KE.
ICN is a pretty small market compared to the amount of capacity that KE offers. It is obvious that KE's business plan is built around high volume connections using their geographic advantage - not unlike what the ME3 do.
Given that DL has reported 25% profit margins on the Pacific during the peak season and have remained profitable when all other carriers have lost money in Asia, DL knows what is in their financial interests and what is not.
It is also worth noting that with the addition of DL's SEA-ICN service, DL is the only US carrier with two ICN transpac flights on its own metal and also has 1 1/2X the capacity to ICN as any other US carrier.
the relationship between DL and KE is still being developed and is not representative of DL's overall mileage program.
You are right that DL has more restrictive upgrade restrictions on partner airlines but they also have more restrictive upgrade policies on DL metal. DL simply does not give away as much of its product as part of incentive programs and yet DL has had the fastest growth in revenue among the legacy US airlines over the past 5 years and is strongly profitable. IN many comparable markets, including across the Pacific, DL has average fares and total onboard revenue that is at the top of or on par with what the highest revenue carriers get. Remember also that a business class seat takes up the space of 3-4 coach seats. If DL has an 80% plus chance of filling those 3-4 coach seats at fares that are close to the average that DL can get for the flight, an upgraded passenger would have to be paying a fare 2.5 or more times the average fare for the market in order for DL to make more money off of one upgraded passengers vs. several coach passengers. Fares of that level for business class are typically available on a confirmed basis.
For a passenger, that might not be what you want to hear. but it is exactly the word that stockholders and employee profit sharing participants want to hear.