If I have to provide my phone number I'll stick with SMS.
It is not giving my phone number to an entity I don't trust.
I don't trust anyone/thing that asks for my phone number without a valid reason. Signal does not have a valid reason. And in the context of privacy that makes signal borderline fraudulent.
Even further, signal uniquely gives facebook and linkedin et al. my contact graph. Why? Because it relies on the contact list. Chances that anyone I'm talking to over signal also has facebook/linkedin/... is rather great.
Even facebook is better in this regard since you don't have to have a phone number to create a facebook account.
Doesn't matter if it's the best thing ever, I could not possible care less about it. Which is a shame because I want to.
It makes it easier to discover from your contacts who already uses Signal by having the phone number tied to a Signal account.
I'd prefer if using a phone number with Signal was optional.
With generic usernames you have nothing. This practically means you have to do some sort identity verification in meatspace beforehand.
There's potentially value in having the option to link two identifiers together, so that people can find you on Signal via phone number or email address. But only as an option.
That's not true. TextNow, and i'm sure others, will just give you a phone number through their app from whatever location you want that you can change nearly at any time. You can use it from a web browser, it doesn't need to be tied to a phone or anything else.
I've personally used it responding to ads and things I'd rather not give my actual number for and a friend of mine used it on a phone for a year or so as their personal phone number when they couldn't afford to pay for regular phone service. That friend actually used one of those numbers to authenticate some other messaging service.
I'm not saying identity is a must or that it's either good or bad, I'm saying that that is what the market is like right now and removing it causes people to see it as a downgrade.
Also, depending on your jurisdiction it might be illegal to provide a number that isn't yours (varying from spam laws to identity theft), just as it might violate the EULA and suspend service from whoever wanted that number in the first place.
Generally, if you want to circumvent doxing yourself there are plenty of methods, but we're talking about mass-market here. The rest uses Jabber with OTR or some form of the signal protocol or maybe plain PGP/GPG crypto. But if we go down the rabbit hole of niches we would have a completely different context to work from.
The market's only like that because there's a lack of alternatives. 'The market' isn't an abstract entity that came into existence out of nowhere. Companies make the choice to provide only that option, people never chose this.
Removing the ability to chat with people using a username and password is the downgrade, that existed long before SMS and phone verification and tying those services to what is essentially an epehemeral number not ever actually owned by you, but the company that provides it.
There's plenty of places in the world where having a consistent phone number isn't really the norm. I've got friends i've saved multiple phone numbers for because they signed up to this or that messenger with different phone numbers.
Phone numbers are useless as an identification method.
I pointed this out in another comment, but because whatsapp and signal use this system, there are random people in my contacts list, whose profile pictures and status updates I can see, but I have no idea who they are because the people that used to have those phone numbers no longer have them and they were reassigned to someone else.
In fact they still recommend doing a physical verification by comparing the safety number.
Their solution is to use the end user's address book as a contact list with users' phone numbers acting as identifiers. The identifiers are mapped to endpoints with some method that is supposed to protect the end user's privacy.
The app could encrypt a list of usernames and store it on a server, but that would affect usability for non-technical users.
That's okay, but does not make the change hard to make.