"One person could overview all code." Not sure that's true. Imagine reviewing the code of the Apollo Guidance Computer, just to take an example from the 60's. The stack of paper containing the source code printout is more than five feet tall: http://skeptics.stackexchange.com/questions/31602/is-this-a-...
Reviewing it would be non-trivial, but in terms of absolute code size, it's not much larger than a moderately-popular open-source project today. It's significantly smaller than React, for example:
