WN has no reason to fly anything but 737's. Keeps maint. costs way down, crew training way down, parts costs way down, service costs way down. On top of that, it fills all their needs and makes them a BOATLOAD of money ever damn day. They fly them full and often. The question might be better asked if they will ever go for the 736 or 738.
No idea on UA 757, but I assume not.
They're not handing trophies out today