neverEnough, When looking at these codeshare agreements it is important to go beyond the press release info. The actual docket filling with the Department of Transportation is where the real info is. From what I read in the Sept. 7, 2016 Korean filing, which I assume is what you are referencing, this is nothing new or extra beyond what has already been approved for years. With one exception:

Delta will codeshare to Da Nang and Nha Trang, Vietnam on Korean and Korean will codeshare on us to Sao Paulo, Brazil. Those are actually the new approval cities that didn't make it into the press release. The rest of the cities were already approved, and have been since the original filing.

Since Vietnam and Brazil don't have open skies agreements with the US or Korea, and we don't have approval to fly to those Vietnamese cities, doesn't it make sense to get access somehow to those passengers to feed into our network?

As you formulate your answer to this, consider the June 7, 2012 DOT approval which almost unilaterally benefits Delta (and mostly Detroit) in the codeshare with Korean. Do you want to give that up?

Bottom line, we should all get the facts before formulating an opinion on codesharing.