Because it was trivial to impersonate someone. The old usernames supported full UTF-8, enabling a wide variety of attacks.
And by paying for nitro, you could choose the numbers too, so step 1. Find someone you want to impersonate, copy their username, and replace one character with a visually identical one, but a different one. Step 2 - pay for nitro, and choose the numbers too.
As usual, malicious actors are the reason why we can’t have nice things.
As far as I can tell, this hasn't been a big problem in practice. Calls can forge caller ID. Emails can forge the From field. It's irritating but isn't exactly halting society.
I am really not sure why Discord made the change. Wanted to prune inactive users? Wanted to sell Nitro Super Mega Premium to jump ahead in line to get your username of choice? Just bored?
But you can’t have a username that looks exactly the same anymore.
I’m in some 140 discord servers. The amount of spam from people impersonating someone popular has reduced drastically since this change.
I wouldn’t call “one click” an investigation.