Whoever Satoshi Nakamoto is, he's an amazing mathematician, and truly a visionary.
Why? Because he was familiar with elliptic curve cryptography?
My impression is that Satoshi was not an expert in anything. He was a generalist. He knew just enough about cryptography, programming, economics, and peer-to-peer networks to make Bitcoin.
He needed the cryptography knowledge to know the properties of ECDSA and hashing functions; programming to actually write the code; economics to know there was actually a need for private, unrestricted money; and peer-to-peer networks to know that this was also needed to make Bitcoin robust.
First off, those are distinctly different professional disciplines; so just knowing what he needed to know from each of those disciplines without screwing it up was no small feat.
Furthermore, there is more to Bitcoin than you seem to be aware. The way the security methods and economic incentives fit together is not simply complex, it's probably perfect. There are subtle, secondary security features in the protocol that; while not making the 51% attack impossible, do contribute to making it generally unprofitable. Most of these secondary interactions are not mentioned in the white paper, but were present from very early on, and were not simply added to the protocol later. Just consider what can be done with blockchain enforcable contracts.
https://en.bitcoin.it/wiki/Contracts