Have you worked on any task related to modeling physical processes, based on my short experience modeling parts of airplane, i believe building something comparable to falcon with 70s technology would have been multiple orders of magnitude harder.
The fact that NASA makes poor decisions today doesn't prove that SpaceX would have been able to create and implement the good plan 40 years ago.
Multiple orders of magnitude? Do the rockets/engines today look anywhere close even to a single order or magnitude better than before?
The computers are much better yes, but not good enough yet to obviate the need for real engine testing and wind tunnels for aerodynamics. Computers in the 70s were already capable of landing autonomously on other planets - as demonstrated multiple times.
The reality is it was possible to build cheap reliable reusable space vehicles with 70s technology.