I'm not getting you.

There's correspondence, there's metadata, and there's phone-ID relationship.

Signal still protects #1 and #2 better than #3. And the way it works, infrastructure load is much bigger than for most other messaging platforms. So it makes total sense they limit registration somehow .

I'm not sure I remember by now what I've read about Signal protocol, but I think the fact of who messages whom they don't have, so it's not just trust.

~~Anyway, if you've read about 90s' mixmaster servers for mail, while Signal developers don't approve of alternative clients, there are libraries and it's possible to make some kind of a mixmaster bot. ~~

I've left this, because it's funny as a good illustration of why they don't want alternative clients, among other things - because I've described a voluntary MITM.