[V_Terranova_Jr]

> then they realized the fuel economy of supersonic aircraft peaks around Mach 3

As I noted in a below comment, there isn't a way for this statement to be accurate without a lot more qualifications. For "equivalent technology level", Mach 2 cruise will beat Mach 3 cruise in passenger seat-miles per unit fuel burn, and below Mach 2 will be better still.

The extremely questionable economics of supersonic commercial flight were realized by technical people far before the 1960s US programs were terminated. Two excellent sources on this topic are by aviation historian Richard K. Smith,

https://www.jstor.org/stable/26802349 , with text freely available at: https://www.thefreelibrary.com/THE+SUPERSONIC+AIRLINER+FIASC...

and the book "High Speed Dreams" which also covers the 1990s HSCT era: https://press.jhu.edu/books/title/8516/high-speed-dreams

There's significantly more to the problem than just managing heat and "solving" takeoff.