What if the account is compromised? Now the spammer is able to do their spams freely on the IP address.
It’s just a hell of a lot easier to black list the entire IP than to try to manually let in small percentage of people who use a VPN AND want to comment or whatever.
Usernames exist for a reason, especially in chat apps. Not having usernames is only going to severely limit your target demographic. And if nobody uses your app does it’s benefits even matter?
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Also none of the comments even bother addressing the actual question.