I strongly disliked that writeup.
(1) The linear extrapolation of maximum speed over time--a trend that is driven almost entirely by the Concorde--to 2,500 miles per hour today is ludicrous. I thought this was a joke when I first read it but the author seemed to take it seriously.
(2) The decrease in speed (which is slight, ignoring the Concorde) has more to do with deregulation and airlines competing on price post-1978 than with the speed limit. Flying somewhat slower is more fuel efficient, and when customers choose flights based on cost being fuel efficient means being competitive. The author makes no mention of this factor.
It may be reasonable to remove the speed limit, but it would almost certainly not result in a broad renaissance of supersonic commercial flight. As a niche for the rich, like the Concorde was, maybe. But the majority of travelers are still going to select cheaper subsonic flights, and PR pressure to fly more fuel efficient planes will also bolster subsonic flight.