ELI5 Fast Planes - why so little progress in recent decades?

ELI5 Explain Like I'm 5: Bite sized answers to stuff you should know about - in a mini podcast

Why haven’t there been more ridiculously fast planes built, like the SR-71 over the last few decades? What was the real reason that the Concorde stopped flying? Did Richard Branson nearly buy it? What about fast planes in the future?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: minhhale, keplar, prometheus2508, shoulderknees, usmanify, thekeffa & kouhoutek.

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