I wouldn't be too overly worried about the 744s that NW uses for two reasons:

1- NW pilots are pretty much top-notch when it comes to evaluating aircraft for problems. I myself have been stuck on a variety of them before takeoff while they, or ground crew, solved problems. While this might be an annoyance to some, it shows me that at least they've got the ball rolling so that a potential problem wouldn't happen in the air.

2- Even if this were to happen in the air, there are only a few patches not totalling more than 3 hours where if NW had to ditch, they would make a water landing. So my point is, more than likely, they'd be able to make an aborted landing over land (I take any chance to speak with pilots when the opportunity presents itself and this account was from one who does the NRT/KIX-DTW routes).

This shouldn't be too big of a problem. If you feel inclined, do track any inconsistencies involving 19/20. The problems should even themselves out soon.