Long term projects often have the problem they are solving disappear out from under them. They become obsolete before they're done. Freeman Dyson famously pointed this out and suggested anyone planning ahead more than five years was fooling themselves (and he meant that in science, too.)

Fusion seems a good illustration of this. Work started on it about half a century ago. Its motivation was "fission will be cheap, but uranium will get scarce, so let's build fusion reactors that have cheap fuel!" Except fission didn't get cheap, the cost turned out to be dominated by the cost of the power plant, not the fuel, and fusion reactors don't solve that problem AT ALL.