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Yes, it seems unfair that you're stuck with a username. It seems secure enough that you wouldn't be able to change the phone number and name associated with the account. The username is more like a "profile pic" or branding issue.

Yeah, or at least have some verbiage making it clear "This is going to be your public username, and you will not be able to change it later."

Yep! Needs to be upgraded for SO many reasons... and I think a personal privacy ability to block certain users from following us is vital. Basic boundaries and privacy.