I feel like email overtook usernames because it was more likely to be unique/memorable. I hate when websites ask me to remember a username (even though I'm using a password manager so I should really just calm down.)
Usernames are no less likely to be unique and memorable than an email. You presumably chose something memorable for your email, so just enter that without the @foo.com bit. There, memorable and probably unique.
Usernames are no less likely to be unique and memorable than an email. You presumably chose something memorable for your email, so just enter that without the @foo.com bit. There, memorable and probably unique.