As a former PM and MM I stopped flying DL in 2002 totally with rare exception. Upgrades were one reason.
Domestically,On a ATL-SJC trip, DL wouldn't upgrade me when I checked in the crown room 45 minutes before departure even though 7-8 seats were still open and I was #1 on upgrade list. "Only 30 minutes before the flight, sir.
My international was even worse. I was on a full Y from So. FL. and could not clear a flight 72 hours out top CDG They only had 17 empty seats still within 48 hours. I finally left a Message in the CEO office that they clear me or I will fly Y on AF home and never fly DL again. They did clear it and when we left 8 empty and 4 NR on the flight.
I was told in 2001 NRT is very rarely available for PMU as they sell out or hold hoping to till the last minute.
Since then I upgrade all the time on AA without trouble.
Please do not say we have only one flight and it is always full. If that is the case add one, its profitable.
I would fly AA, CO, NW etc to NRT as they are all better on overall service and availability than DL.