[QUOTE=themicah]Umm.... How did it feel like the 1970s? The very first 757 test flight wasn't until 1982, and I don't think they entered passenger service until 1983, or NW's fleet until 1985 (correct me if I'm wrong).
QUOTE]

You are correct


To the OP:

While "often" is probably subjective, NW doesn't typically swap equipment unless there is a need. In my opinion, NW doesn't change equipment often.

Some of the 757-200's were acquired within the past 5 years, so I suspect those still have quite a bit of life left in them. If you're unhappy with that plane, either pick another routing so you can avoid it, or check back in 2020, when those planes may be gone.