To some degree that's the case. The Falcon 9/Dragon do very much get a benefit from modern technology. Better machine tools, high-speed digital communications and onboard computers, better material science, the ability to run computer simulations of different flight profiles or to model the operation of a rocket engine, etc. However, in another sense the Falcon 9/Dragon represent very primitive designs. Indeed, 1950s or 1960s designs even. A simple cylindrical 2 stage LOX/Kerosene booster in combination with a simple frustrum shaped manned capsule. These designs have heritage going all the way back to project Mercury.

Yeah, and guns are vastly more reliable than rockets or missiles, too. Sometimes old and simple is the way to have high reliability, sacrificing performance.

The space shuttle was a poor design in many respects even by the standards of the 1970s as described in Feynman's Challenger crash report http://www.ralentz.com/old/space/feynman-report.html

Good design does magically appear as time goes by but is a result of a sound design and testing process. Fortunately, I am sure SpaceX is aware of this so they will most likely get a much better track record than the space shuttle.

why?

Because we have new materials, new technologies, and 40 years of experience with space plane design and operations, that's why.

The U-2 spy plane is in service after the retirement of its original replacement.

The B-52 is still a heavily used part of the US heavy bomber fleet and current plans are to keep it in service until 2040 and perhaps beyond (which is longer than the B-1b is planned to be in service).