Phone numbers are recurring costs. And to keep a truly private one you must keep paying without ever disclosing personal info and that is really hard. Signal is a privacy nightmare for long term use.
https://support.signal.org/hc/en-us/articles/360007059792-Si...
Not only that, but its a unique identifier people generally have already had and generally have already shared and historically been OK with sharing with people they want to talk to. That's a part of the reason why Signal originally chose that way of finding contacts, people were already connected in that way. It makes on boarding people massively easier and greatly reduces the friction of people actually using it. A messaging platform is pretty useless if I can't easily find my friends on it.
> And is terrible UX, imagine having to add your contacts new numbers every other week
Practically nobody is getting a new phone number every other week. And once again, if you are the kind of person getting a new phone number every other week, I'd agree Signal probably isn't the platform for you.
If you don't have a phone number or your number changes all the time, I agree Signal isn't the choice for you. If you already have a phone number, are OK with what having a phone number means in terms of privacy, and that phone number is pretty stable, then Signal isn't a bad choice to use to message on.
It does mean theoretically some large organization (like a government with a warrant) can potentially see "John Doe has this phone number, this phone number is related to Signal, therefore John Doe possibly uses Signal", but personally I'm not too worried about that tiny bit of information leakage. Besides, with enough effort one could probably ID that looking at internet traffic patterns unless you're really that paranoid about controlling your network routing. Especially when that means I'm able to actually convince family to use the platform, as they're used to just looking up people by phone numbers and don't want to have to deal with managing yet another unique identifier on yet another platform. If they had to register another account and manage yet another identity, they wouldn't use it, and thus I'd be stuck just talking SMS with them which results in worse privacy outcomes for our conversations.
Getting and maintaining an active phone number privately is indeed quite hard, partially by governmental design.
Signal only requires occasional/rare proof of control of the registered phone number. It also has very little visible data the provider can access on your account, even if they had a reason to assist in breaking your privacy by look it up from the phone number. Without Signal foundation direct support, the phone number linkage to your Signal account is completely opt in by you only.
So in terms of privacy, Signal is actually very good about the phone number and leaves it mostly to you how public you want to be about it. They're primarily using it as a finite controlled resource to limit how easy it is for people to spin up arbitrary new accounts. Other projects might use some cryptocurrency junk that effectively equates to paying for accounts, but Signal uses what you probably already have.