Hey, at least thanks for having done your research on the topic :-)

Re: “Signal technically cannot know your social graph” is more of “we, Signal, have got the information in our hands but we swear not to look at it”. Essentially, your device is sending the data to Signal, and then the matching is done in a “secure enclave”. One problem is that this step could totally be bypassed without your knowledge or consent. A second is that the technological underpinning of it (Intel SGX) has known unpatchable flaws. A third is that even if the build-up of your social-graph isn’t stored initially, it can eventually be inferred from your usage patterns. A fourth is that even if you find good reasons to trust Signal today, they offer no definitive technological guarantee to enforce it in the future (the deal can change at any moment, being a non-profit isn’t a guarantee either).

I will also say that, in a decentralised communication system, you are reliant on every party you communicate with, and the tools they use, to not expose such data about you either. It’s not a panacea.

No, in a decentralized system, you elevate your service provider to the same level of trust that you do today with Signal (with E2EE and maths taking care of the rest). The gotcha lies in the fact that you can be your own service provider in this case, or that you can establish other means to trust them (contractual, legal, moral, … obligations, that’s up to you). And in the fact that changing service provider doesn’t mean relinquishing all your contacts, histories, data, clients, etc…

it’s utterly ridiculous to claim that this is not a way forward

I don’t disagree that Signal has some appeal over WhatsApp today. I only disagree that it represents a significant-enough step forward to justify having people massively migrate to it. From experience it is a doomed service that will deceive its users eventually (by design), and will cause more harm down the road (triggering another unorganized rush towards even worse services like Telegram) when it ultimately gets to this point. If you ask people old-enough to have known and used WhatsApp in its early days, they will depict a picture about as rosy as the one you paint today for Signal. All that to say, once again, that nothing is eternal. Especially in today’s extremely consolidated internet (like, who would get in the way of Meta, Alphabet or Microsoft buying off Signal if they ever want to?).