Without turning this into a thread on aircraft specs, sectors at the edge of range max tend not to be great economically. Strong predicted headwinds can require less than full payloads (or technical, ie: fuel, stops that make the flight expensive and late). Reduced payload = empty seats, meaning the remaining pax have to pay high fares or DL loses money.

This is why it makes sense for Delta to improve their relationship and connections with KE in Seoul.

Will Delta eventually take the 787s and do some routes like this? Maybe, but I'm not convinced TPE would be the best use.