How does a network admin debug a binary protocol for which no dissector has been implemented/merged into core for Wireshark, and no decoder has been written for tcpdump?

It's obviously doable, but it's very painful.

Not as absurd as you'd think. You don't want to debug the protocol itself, but you want to be able to easily read what messages were exchanged.

I get the rationale. But I think it's weak, and this entire post is lots of fluff around that core rationale. (I've been writing extensible binary protocols back in 1988 - and it never struck me as particularly difficult even back then.)