NW's DC-10-30s are a mixed fleet. Some have less than 40,000 hours on them, while others (soon to be retired) have 125,000. The MD
-10 mods would be mostly cockpit mods, saving you the FE
pay, as well as lightening the airframe by deleting equipment and the FE
by 1000 pounds, thereby increasing payload. Winglets are NOT part of the MD
The main reason for not upgrading would be that the DC-10 will not be in service long enough to realize any savings from the mods. The older airframes will be retired at their next heavy checks, and the younger ones will be phased out en masse once their fleet type reaches a small enough level. The younger ones would have a lot of airframe life left in them, however there isn't too much of a market for used older aircraft these days, so the costs would probably not be recovered in resale.