twistedenginestarter,
Up to the tropopause the temperature of the air decreases, offsetting some of the loss of the lower pressures. Above the tropopause (about 36,000') the temperature stops dcreasing (and in fact begins to increase in the mesosphere above ~65,000') so the specific fuel consumption of the engine begins to increase above the troposphere, and thrust decreases.
Concorde is designed to operate at supersonic speeds. The engines need to have subsonic air at the compressor inlet, and this requires a different intake to that on fan engines fitted to subsonic airliners. The engines are not as efficient, but the speed increase is worth the loss.