I don't think they solved the impersonation problem. You can still look exactly like someone else, and you can only figure out that person X isn't who you think they are upon investigation. Most people don't investigate anything except the display name and profile picture, so are still vulnerable to impersonation.

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?