Why? If someone wants to use a phone number for their identifier, fine. But make it optional. If I join Signal with another alias (free-form string perhaps) then they could just add me via that. Not really harder than using my phone number.
Network effects.
If you’re only findable via custom handle versus the phone number your network already has, you’ve reduced the network’s value to your contacts. Put another way, if I join a messaging service and it says two of my contacts are on it (but many more may be), that’s close to a non-starter.
That’s a private gain at the network’s cost. For a challenger, their decision to bar that albeit limited form of free-riding is perfectly rational. For a dominant network, I am much more sympathetic to the call for anonymity.
Network effects make these virtually indistinguishable. If a network is unusable it doesn’t help anyone.
There may be an argument, however, for charging for the opt out.
Even if that was the case, " it's strictly less usable than WhatsApp" is not true, because other messengers can provide other quality of life features.
" That means in practice, it's unusable." is not true either. Remove "Status" support from WhatsApp and you get strictly less usable than WhatsApp. Still, it is not unusable.
Signal is pretty close to being strictly less usable than WhatsApp btw.
And everyone with a phone has a phone number.