What happens when the handshake blockchain forks (say, because the developers can't agree on a change), which of the two 'trustless' forks am I meant to trust from that point on?
Ultimately you'd have to decide which one you want to treat as the canonical chain. There would be two (or more) parallel histories, and users would have to figure out which one they wanted to follow.
By default, all users would go along with the original chain. To follow the chain with the updated rules (whatever rules the developers couldn't agree on), a user would have to manually opt-in to that updated set of rules.
