Signal's requirement for phone numbers has allowed it to surge in popularity by allowing it to take advantage of people's already established contacts. They do this in a privacy respecting way[1].

Simultaneously, Signal is trying to raise the cost of accounts by requiring phone numbers. Although spammers can get mass amounts of phone numbers, it will at least raise the cost. Email 0 cents, phone # 10 cents–there will be less spammers with phone #s.

I don't think we'd have to worry about the spam if people only used usernames instead of phone numbers, because it would be massively harder for spammers to find your account and message you. But, with usernames, you don't get the contact discovery that allows for growth.

[1] https://signal.org:8443/blog/private-contact-discovery/

That's all nice but it's also the defacto option for non technical people who need state actor resistant security and if they can just be subpoenaed to get the verified ID of the account holder that defeats the entire use case