Persistent data structures (like git) are way harder to update than non-persistent structures (like ephemeral network packets).

Hypothetical WireGuard v2 packets will simply not parse as WireGuard v1 packets; they can't be decrypted as WireGuard v1. And vice versa. It's therefore trivial to have both protocols coexist, even on the same UDP port, without any negotiation whatsoever. You simply need to discard packets that don't decrypt, which is what WireGuard already does.

It's dangerous to include a downgrade-capable negotiation in your VPN protocol. I'm glad they left that out of WireGuard.