I agree that there are a lot of design flaws in the old protocol, but the problem is, there is no logical path forward for ActivityPub implementations to support both versions at once.
And what happens if it changes again?
Why did the new draft not acknowledge that it broke current implementations and provide this advice?
Why should we not just fork HTTP Signatures and introduce our own X-ActivityPub-Signature header?
It is possible to make changes without breaking the consumers you asked 6 months ago to write into the IETF letting them know about our usage. Instead, as a thank you to anybody who did that, we get hit with a draft that completely breaks us.