How does limiting the timeline to 5,000 years enhance our ability to predict the future?

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Good point, maybe a better way of phrasing it would have been "It's virtually impossible to predict which tools will _not_ be useful" :)

Fast Fourier transforms, complex numbers, complexity classes, etc. are clearly useful. It's much harder to rigorously claim that a result will _never_ be practical or useful.

Just to be clear, I think it's fair to say that a result isn't _immediately_ useful. But who can say whether or not it will have a more practical application down the road? Only time will tell.