The real problem is that the mission the SR-71 was built for doesn't exist anymore, and nobody else with a need to go that fast has the money to back it up. We could build a successor the SR-71 that would go even faster and further and have fewer weird issues like fuel leaking out constantly, but it would cost a lot more (even inflation adjusted) than the SR-71 did and fulfill a mission that nobody was asking for.

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.