Evolution: Survival of the fittest.
An often misunderstood expression.
Fittest does not mean the most athletic or physically strong, it means fit for it's environment.
When the environment changes the animal/plant/aircraft needs to evolve to best fit that changing environment.
Living organisms take many generations to change due to the randomness of genetic mutation. Aircraft design, in comparison, changes relatively quickly as new technology and ideas develop.
The environment changed in the seventies, fuel prices exploded. The 747, and continued lines of fuel efficient wide bodies thrived, Concorde only continued due to political will. If the price of fuel was still $20/barrel Concorde (and probably a couple of successors) would be going strong as it would still "fit" the political and economic environment. (The greens may have put a bit of pressure on though
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Simples eh?