The main concern I have about the Discord usernames debacle that I haven't seen addressed:

CAMPING

• People will absolutely use this as an excuse to steal the usernames of people they don't personally like. Not being able to change your username to avoid someone who is stalking you because they will just take it.
• I give it like, a month, before there's servers for trading the shortest possible noun usernames you can get. Money will be involved. (See Neopets, with its "Real Word" and "Real Name" and "3 Letters" pets)
• "I got the username pikachu because I'm the real pikachu. I'm the person who deserves it the most." "GRRR, I HATE THAT FAKE PIKACHU WHO HAS THE PIKACHU USERNAME! I THINK I WILL SEND THEM HATE MESSAGES!"
• Understanding that usernames are not unique at least gives the average person a motivator to double-check if they're really being messaged by the real MrFeast or whatever.
• Speaking of, roleplayers camping on internet personality usernames. I could give no less of a shit about MrYeast but you know the drama icebergs are going to be terrible. Especially once we get into the people who do have one-word usernames like Creamwastaken and "is it fair that he gets to have it before anyone else?" and blah blah blah this is just going to suck.

All of this is based on real events I have seen personally. Unique usernames are and have always been one of the largest sources of drama on any platform; Discord managed to elegantly sidestep this issue for nearly 10 years.

[edit: on June 7th I would find out someone had camped on the username Markiplier.]