Re flying slower:
probably true if you look at the average cruising speed in 1990 - but then again there were more aircraft with more wing sweep as a proportion of airliners back then. Majority of airliners in 1990 were obviously designed in the 60s and built in the 70s and 80s.
In 2020, with the massive growth in aviation since the 1990s, the overwhelming majority of the airline fleet are based on aircraft designs conceived post 70s oil crisis and built post millenium.
The transonic regime is increasingly unfriendly in an economic sense as you close in on Mach 1.
The Citation X gives a few clues as to the requirements for operating in this flight regime.
