If there would forever be only one encryption algorithm in use we we wouldn't need most of the stuff.
Coincidentally, this is why a protocol like WireGuard is much simpler than OpenVPN. It's a clean-sheet design, so its developers had the luxury to design it with hardcoded modern crypto primitives without any revision or extension. Protocol negotiation is skipped entirely since it's often error-prone and suspect to attacks...
I think it's basically the plan.
Even with protocol negotiation, replacing an existing crypto algorithm would be a breaking change anyway. Furthermore, it should be possible to preserve backward compatibility using a reserved field without a heavy-weight, full-fledged protocol negotiation. Entering a different code path if the message is tagged "version 2" should be enough.