1. It isn't satoshi's work that made it so. I think it was Amir that first raised the preference to have signaling and miner buyin, and formalized by Gavin: https://gist.github.com/gavinandresen/2355445
2. On your day-to-day you need 100% consensus with everyone you're transacting with.

It's an elegant way of managing consensus, but also disagreement.