Airspeed isn't valid below a certain value (the difference between total and static pressure is too small to be dependably resolved). Some Air Data Computers use 30 knots (that's what's used on 747/767 that I work), presumably on the 737 they use 45 knots.
When they went to the NG, Boeing didn't do a complete redesign the 737 wing, they did a 'reloft' where they changed the aerodynamic profile but not the basic structure. Since the 737 wing dates back to the 'short haul' airplane concept of the 1960s, there really wasn't much they could do to the wing sweep within the design constraints of keeping the same basic wing structure.