I wouldn't say Concorde was ever "unsafe," but as ConcordeBoy points out, the tire issue was a serious design flaw not truly corrected until after AF4590. Pictures such as the one above show that a frightening degree of damage could be and was inflicted by bursting tires, and it's a little disconcerting if the good people at BAe and Aerospatiale could not imagine a major incident resulting from such damage. Continental was negligent with their faulty repair work, but past tire busts showed that Concorde didn't need runway debris to get into trouble. All aircraft, especially one so revolutionary, have design problems; it's just that this particular problem should have been fixed years before the crash.
Despite the highly inaccurate and sensationalistic Discovery "Anatomy of a Disaster" program, which apparently did not consider that simultaneous loss of two engines on one side of the aircraft could case a yaw, the official report concluded the spacer absence and slightly overweight takeoff condition did not materially affect the results of the accident. The report states "In theory, the absence of the spacer could have instigated an asymmetrical trajectory, tyre overheating, and slower acceleration than normal. Study of the marks on the runway as well as calculations of the trajectory and accleration made on the basis of the data from the flight recorders show that this was not the case" (126.96.36.199). Several evidentiary facts are presented, including that the aircraft stayed on the runway centerline until after the tire burst and thrust loss. Absence of the spacer constituted negligence on the part of AF
maintenance, but it did not contribute to the crash.
At maximum gross weight (408,000 lbs.), V1=139-162 kt (150 kt was selected), Vr=199 kt, and V2
=220 kt. The report found that "For all these values, the influence of an increase in weight of one ton was examined and found to be negligible." In any case, Vzrc with two engines out and gear extended was more than 300 knots; with three good engines, it was 205 kts. Given the condition of the aircraft, with one failed engine and one intermittant engine and the gear stuck down, there was no way stable flight could have been maintained.
They key factors were loss of thrust due to FOD and hot gas ingestion and damage to control surfaces and airframe components due to fire. Even if engine two had not been shut down, the aircraft probably would have been lost because it become uncontrollable.
Keynes is dead and we are living in his long run.