I think 9/11 was coincidental and unrelated. The important trend in 2001+ was the growth of telecommunications phenomena that reduced travel demand. I am not sure if the sonic boom was nearly as big a problem as it was made out to be. I think we can do some cursory math.

If 30% was at 50% of the speed, the total distance covered would be 85% less, adding about 20% flight time increase. Something that normally takes 8+ hours now take 3 or 4 hours. Did this kill the concord?