The phone number requirement is a killing feature for me, makes it useless. I wouldn't put it even close to the UX bar Keybase has for me.
I think the idea is that privacy != anonymity. Signal provides the former, but not the latter.
It's tough. I think that usernames could become messy, but I also think it'd be amazing to anonymously tip a news reporter via Signal, but at the same time the latter would not be as safe as Tor etc.