I have always thought Concorde was grounded after 9/11 due to pressure from the US government (remember the first proving flight following the refit turned back mid Atlantic on that day) because a high-jacked Concorde could out fly the US missile defense system.
A short diplomatic exchange along the lines of "ground Concorde and we'll give BA and AF some more routes or else..."
It did not make sense grounding an A/C that had a near perfect safety record and had just had millions spent making it even safer.
I have no evidence to back this up except the general paranoia that existed in the States following 9/11