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
From my understanding the aircraft was terminated operationally due to the inherent costs in the re-engining program and other modifications.
The fact that the TC was cancelled was nothing to do with costs, safety, or operability, just an assurance that no one else would be able to use one ever again.
stopping using an aircraft doesn't canx the TC, once the TC is canx it can never be flown again (outside of experimental with administrator approval) without undergoing the type certification process and meeting the requirements as set forth by the administering authority at the the time.