I'd argue the opposite - it's harder to get away with impersonation. In the AIM situation, users go in "knowing" that usernames are unique, so if you see a mes

I'd argue the opposite - it's harder to get away with impersonation. In the AIM situation, users go in "knowing" that usernames are unique, so if you see a message from user abc123, you "know" it's that person. Which means user abcl23 ("el" instead of "one") can impersonate the real user. In Discord situations, (1) the user is more likely to know that usernames can be reused, since they weren't required to make a unique one to start - so they can be more cautious of impersonation attempts, and more importantly (2) the Discord devs know names can be reused/abused, and can build better mitigations if they so choose to (like e.g. putting a disclaimer at the top of any message from a user you've never contacted before, etc.)