Earlier quoted context omitted.
It's down to the laws of physics. They still fly at around 80% of the speed of sound, as that is close to the so-called "coffin corner". https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics)
It's unrelated to the coffin corner. Planes in the ~80s were flying NY London faster than today -- the fuel savings generated by flying slower are what is driving the decision.
Yeah, you are absolutely right :)