This will probably never happen. One of the reasons WhatsApp blew up is because using a phone number as your source of identification means there's much less friction in the signup flow. No username/password to create and your social graph is already there in your contact list.
My mom was able to get our entire extended family on Signal without my involvement, which is a testament to how easy that is.
They also blew up because it was also quite decent SMS app, so you just had to install Signal and use it instead of your default SMS app. All your messages are there, you can continue to communicate exactly like you did before, except that now, if the other person also has Signal, your messages are encrypted.
They stopped doing that (and I uninstalled Signal as a result), so they can also stop with the phone number thing, in fact, it would make more sense than with the current situation where Signal needs a phone number but doesn't use it (except for registration). I could even reinstall Signal if they do this.
In the short term it will, and quite possibly in a long-term also, but if you were going to fully make phone numbers optional, I'm pretty sure this is the first step you would take. At the very least it sure looks like they're starting to build the possibility.
Nobody is demanding them to stop supporting phone numbers as identifiers/verification methods.
I'm not mad at all if somebody prefers using their phone number and not having a password for a service – just give me the option to use my email address and/or a username.
There are too many "phone number only" services out there these days.
My mom was able to get our entire extended family on Signal without my involvement, which is a testament to how easy that is.