If Signal-the-app did a WhatsApp and started blocking forwarded messages, or blocking messages that contained a blacklisted domain etc, I think very little would happen and it might take a while for people to even notice. You're assuming it'd be noticed immediately and everyone would find out nearly as quick, but I see no evidence this is true. There is no organized effort to do this type of auditing, let alone make it sustainable for the long run. Even if there was, so what? By the time I somehow find out (how? reading HN?) my message history is already gone.
"What I don’t follow is why you think it requires multiple clients."
I don't ...
"TBH, I didn’t read this part of your post that closely"
... which might explain why you keep mis-characterizing my position. Please RTFA and then opine. The section on threshold signed auditable builds isn't even long or complicated, it's a few paragraphs at most.
With the proposed software update scheme you can at least get closer to the goal of a single-client-single-server but genuinely encrypted system, that meets the social goals people actually have for encryption. It's a form of decentralization that doesn't require everyone to re-write the same code several times, so should mostly dodge the product management and iteration speed issues that causes Signal to reject federation.
It isn't as good as having multiple clients because the auditors can only reject new changes, not actually improve the privacy of the product in the way that a competitive market of clients might allow, but the tradeoff is the central authority can add features more consistently as there's no client capability skew.
"Well, no, I disagree. As a prominent example, Certificate Transparency “Noticing”—aka accountability—is extremely useful."
CT is hardly used. I've never heard of a single incident in which a website operator found a bad cert by monitoring CT logs. If there are any such cases it is very likely to be a big tech firm and not a "normal" SSL user.
Regardless, CT is if anything a better analogy for my own proposal. The actionable outcome of a CA breach detected via CT is that browser makers revoke it and they stop being able to issue certificates. It happens behind the scenes and users don't have to think about it. Imagine if browser makers never revoked any CA and people just had to tell their friends that some padlock icons were trustworthy, others weren't and they shouldn't browse to websites that used the leaky CA. Good luck with that.