I heard somewhere (Can’t remember where) that NW’s 787 would be used to replace their DC-10 fleet. I can’t see Northwestern replacing their A330s with the Dreamliner considering how long these aircraft have been in service. It wouldn’t make any sense!

It would if the 787 did a better job for less money.
NWA only pays $80m for each a330. So its not all that much lost if they sell em on.

Having said that… I think the 787s will augment the A330 fleet, replacing the DC-10s as you say