Note that overflights of the USSR started well before the U-2. As early as 1946, the US was flying reconnaissance planes (usually modified strategic bombers) over the Soviet Union. The U-2 came about because the Soviets began to gain the ability to intercept and possibly shoot down those flights.
That's not really accurate.
It was never intended to be undetectable, just unhittable, because it flew so high that missles and guns didn't have enough velocity to even reach that high.
And it was introduced before the SR-71, and is still in service (the SR-71 was retired in '98/'99). If you compare the price differences, one U-2 costs ~3% of a SR-71, though that's excluding maintenance.
Also, the history (70 year old plane) has a great look into military and intelligence collaboration with American industrial design firms, like Lockheed Martin.
The "grown up versions" of this are probably all classified at this point. The declassified modern capabilities give some hint to what the CIA is able to do. The ARGUS-IS is able to surveil, in high definition, in real time, entire cities while tracking thousands of targets, identifying them as people by their shadows, synthesize data with other sensors (cellular, wireless surveillance, etc), all without being detected.