I don't know what the real reason is, what I said was just something that popped into my head. Another comment mentioned spam-prevention as a reason (by making it infeasibly expensive), and that actually makes more sense. Honestly, there probably isn't just one reason, but a cluster of tradeoffs.
> Isn't it better to have a larger pool of people with whom I can communicate securely using Signal?
IMHO, the number people who care deeply enough about the phone number thing to boycott Signal is vanishingly small; not even a rounding error. Sure they're loud on HN or maybe even Twitter, but giving tiny but loud minorities whatever they demand is bad policy.
What I meant was: "it's better if I can use Signal to communicate with people even if all I know is their email address".
I'm mostly just confused because this is being presented as a technical limitation: using email addresses would supposedly "require Signal to keep a database of contacts serverside". I don't understand how or why that's true.