Autotrim
It wasn't quite as simple as that. The fuel transfer system really fixed long term problems like getting the elevator trim broadly at optimum throughout (and really at optimum in cruise of course). The Mach trim/autotrim really worked on a shorter timescale to maintain stability at constant CG.
Sure the autopilot made it superfluous to some extent, but to certificate the aircraft it had to be conventionally stable when flow manually, and applying a nose down command to get a speed increase is a basic airworthiness requirement for all aircraft.