It almost certainly won't be a single major technical issue that ends Concorde, rather increasingly high maintenance costs which will one day exceed earnings.
When that day arrives will be known well in advance by the airlines, and a phasing-out planned.
Airframe life is determined by what is called a Reference Flight, a flight with 170 Ton or more take off weight is 1 RF, less than 120 ton (like a training or ferry flight) is a 0.5 RF.
With the very good condition of the airframes found at the first series of major checks in the early 1990's, there was an extension from the original very conservative 6700 RF life, to 8500 RF.
BA aircraft have more hours, the oldest flying BA Concorde should reach 8500 RF's in about 2007-8, depending on future flying rates.
However, an extension to 10000 RF is possible, again depending on what (if any) airframe mods are needed.
But the Major checks needed to assess this, (when the A/C reach 24000 hrs), are not certain, if both operators commit to these checks then the costs drop, currently AF have not, (though their fleet is much further off 24000 hrs), BA at present do not have the manpower to do a Major, so a reversal in current policy towards resourcing Concorde at BA will be needed, current problems in this area are more to do with post Sept 11th problems, but BA took a hard-headed business decision last year to still go with the re-launch, after checking running costs with a specialist outside consultants company.
The more Concorde earns, the greater chance of better resourcing in the future, the last few months have seen some very encouraging figures in this area, which also pays off the relaunch costs quicker.
The best guarantee of that is for Concorde's current loads to continue, with more services, adding flights would be offset by bringing the 6th BA A/C G-BOAB, back into operation.
But for this reason, don't ever expect any major charter operation to re-emerge.
One potential problem for BA is that most of the fleet have around the same number of hours, except for OAF and OAG, which have fewer hours, but maintaining just 2 A/C is never going to be a viable operation.
But RFs are not the only issue, maintaining personnel current in Concorde's unique systems is not easy, it's very untypical of modern airliners.
Costs are high when it comes to replacing for example avionic items.
So far, the wiring has stood up very well to the rigors of Concorde's operation, the intake systems were rewired in the early 90's, that's an example of a major maintenance job that could yet occur.
Support from the manufacturers is another issue, it's expensive and often protracted.
If one operator retires Concorde, then support costs for the other would soon become unsustainable, even if some or all of the aircraft/spares were given to the other.
As BA and AF started together, and relaunched together last year, one operator pulling out is not likely.
Co-operation between BA and AF on technical and spares support, while always strong, has increased recently.
Weighing all these up, I would expect Concorde to operate until the 2008-10 area.
If additional services (e.g. BA003/004) with good loads happen, and BA 'man-up' their operation, do the Major checks, find no major issues, the authorities agree an extension to 10,000 RFs, then you could indeed see a BA fleet of 4 or 5 aircraft operate a BA001/BA002 operation up to about 2014-16.
But I would not bet on that, however the aircraft has defied the grim reaper many times already!