An exact replica of Falcon 9 was impossible 40 years ago, of course. But a replica with 50% lower thrust, 10% lower ISP and 60% it's payload capacity for a similar inflation-adjusted price was certainly possible.

It wasn't the computational revolution that made SpaceX possible, the basic technology is more or less the same as it was in the 60s. The Merlin engine has it's roots in the '90s NASA Fastrac design, evolved for performance and reusability.

SpaceX's success relates to things like market incentives, lean business practices, a fail early and iterate quickly attitude as opposed to "too big to fail" public projects, and so on.