If you're interested in the dead end of space planes, Tom Wolfe's "The Right Stuff" covers how the program lost out to the Mercury system.

There is just so much wasted potential in the early 70s. It's really the death of optimism in human progress.

Not the death of optimism, just the winning of the NASA bureaucracy in keeping the organization size by pushing the "cheaper" space shuttle.

With the benefit of hindsight, flying into space with the X-15's successors would have happened if there was not a need to beat the Soviets quickly with rockets.

Also throwing away the Saturn rockets after around 20 launches was just silly.

Correct on all counts. Turning those remaining Saturn Vs into lawn ornaments was criminal.

I believe that the apotheosis of human achievement was likely around 1974 or thereabouts.

The rest is just incrementalism which is more than cancelled out by serious backsliding all the way around.

The X-20 looks like the Virgin Galactic's SpaceShipOne (and SpaceShipTwo).

You got that precisely backwards, heh.

During the X-Prize launches, I observed that what Burt Rutan had done was to build himself an X-15, and then build himself a B-52 to launch it.