Well, you’re giving a not so great example there.
I don’t know anyone that uses WhatsApp because of their encryption system or privacy claims. Whereas I also don’t know anyone who would keep using signal if not for their encryption and privacy claims.
In every metric I can perceive signal is just playing catch-up with WhatsApp or telegram features. The only winning feature is privacy and encryption. So, your argument that:
> In reality even if some people checked it'd make no difference
Makes no sense, if one person could break the trust on signal, it would be absolutely dead. No one would keep using a poor substitute with barely any user compared to the mainstream chat services if not for privacy and trust.
OP's point is that the change in forwarding was practically equivalent to (partially) defeating the encryption.
I think the argument is that, previous to this Whatsapp forward counter rollout, one would assume significant hits to credibility would block a company/org rolling out such a thing in a client. And that hit apparently didn't happen.
This seems to miss two things.
First, "networks are very sticky" is the reason Signal uses telephone numbers as identifiers. Your network is not tied to Signal, or even to your contacts having smartphones that can run apps of any kind.
Second, the target markets for Signal and WhatsApp are quite different. WhatsApp users more often than not don't care about privacy. They care that everyone they want to talk to, including possibly critical government services in many countries, requires them to use WhatsApp. So if WhatsApp stops encrypting entirely, I'm not sure very many users care. Signal is much more likely to be used by people who actually care about privacy and security and is much more likely to be abandoned if it breaks the promise of maintaining those things. It's even designed to be easy to abandon, whereas WhatsApp very much is not. Leave WhatsApp and there are a whole lot of people and services you just can't talk to any more.
In reality even if some people checked it'd make no difference. Networks are very sticky. When WhatsApp broke the encryption by allowing Facebook to detect forwarded messages, there was no uproar or sudden loss of trust. There was no mass migration away from WhatsApp. One day there was privacy. The next there was less, because having made its fortune on the back of viral sharing in social networks Facebook decided that maybe there was a little bit too much social going on and not enough hierarchy. And there's no possible fix, beyond "convince everyone not to use WhatsApp" which isn't going to happen anytime soon.
That's why I suggested a globally distributed group of auditors creating threshold signatures. It should be backwards compatible with the app stores because the resulting signatures are/can be indistinguishable from a regular signature. I have a Shoup RSA implementation lying around in my mail archive somewhere.
Anyway, encryption is about meeting social goals, not merely doing maths for the sake of it. There's no way to meet the underlying social goals here without decentralization - the fact that this is hard is irrelevant (even though I fully agree with Moxie on the problems around iteration speed etc).