A node (client or relay) that doesn't upgrade will see these as broken invalid events. The signature will not match the pubkey field.

In the current NIP-26, at least the events are valid and people can see them, they just don't have a clear association between the signer and the delegator.

So I think this is a worse experience for nodes that don't upgrade.

If every node upgraded it would be fine, but I'm not sure it would be better. Both ways of doing it require code to update either it's signature verification or it's method of associating an author, neither being a harder ask IMHO.

BTW the delegation proof doesn't have to be replicated in every event if we wanted to save space, it could be a separate event.