They could do this. The challenge becomes that unlike NRT, only DTW has flights from the mainland to NGO, not ATL/SEA/etc. This means that the NGO-MNL leg is comprised entirely of Japanese based pax + pax on the DTW-NGO leg. So unless you are selling a lot of onward tickets sold to Japanese pax, if DL is going to operate DTW-NGO-MNL... they might as well just operate DTW/SEA/orLAX-MNL direct.

Swap out DTW for SEA on KIX, and MNL for SIN and you can make the same arguments for other shifting of the 5th freedom routes to NGO or KIX.

Basically, it would require a combination of lighter loads on the NorthAmerican-KIX/NGO load, coupled with good Japanese sales to SIN/MNL to make it worthwhile. But maybe that was the grand plan all along, considering the HKG drop and SEA-KIX add. NGO/KIX provide the opportunity for two onward routes, and DL chose MNL and SIN via SEA/DTW.