Commercial passenger jets aren’t so much getting slower as they are happy cruising in their Goldilocks zone of around Mach 0.84 to Mach 0.85 for widebody aircraft and Mach 0.72 to Mach 0.78 for narrowbody aircraft. The push in the 1960s and 1970s for supersonic passenger travel mostly failed or fizzled, with only 14 Concordes (and some failed Tu-144s) to show for it.