This causes all kinds of pain irl, it's not really a good example. IRL has the same issues non-unique usernames do. It's almost irrelevant in a small community because of the number of members. It becomes a problem in large communities because you can't find someone based on their name.
Discord's issue was downplaying the discriminator so much that a lot of people didn't realize they existed. That if you wanted to add someone, they weren't uniquely identified by username alone.
They probably could have solved that by making the discriminator more prominent early on in the app's lifecycle.
The issues are.. trivial at most. If you can't find someone based on their name in a large community, that can't cause any problems, because... how did you learn their name? If you know who they are, you can find them. The situation where you're given a piece of paper and told "find a specific person by this name, but not anyone else by the same name, within X time period or you lose this game" doesn't actually arise "IRL".
It's that last point there, that Discord seems to want to tackle... We will have to wait to see how, but I hope they don't ruin the server paradigm and aim to become a subreddit type ecosystem.
What? Why?
Names are never unique. That's not how names work. Why would usernames be different?