I have no idea why they decided on this system - there's definitely some improvements such as decoupling username and display name, and making the username case insensitive, but unique usernames and prestigious usernames is just such an awful system.
So many things had solved this issue as far back as the early 2000s, but seems everyone is insistent on using it.
I couldn't get a decent username on Discord either, so I just claimed a game name as a meme (pso2ngs) until I can think of something I'd actually want.
One day Discord might go the way of the Reddit. Its the same. One company holding a lot of user generated content. Difference is that it isn't well organised and is locked away behind an account wall.