We can't do that kind of engineering anymore. Computers have made us ... lazy? That's not really the word. With so much computing power at hand, we just don't have the pinpoint focus that it would take to make another SR-71 and not hobble it with over engineering and feature creep. Or even just being able to let go of a nagging problem. For example, a proper sealant couldn't be found to keep the fuel from leaking so they just said whatever and let it leak fuel until it warmed up. Nobody would do that today.
Computers didn't make us lazy, but the Git-R-Done mentality of the old projects got gobbled up by the bureaucracy in all of our big defense contractors/aerospace companies. It's super hard for a Kelly Johnson type to fend off thousands of middle managers all trying to justify their position.