Their stated reason[1] for doing so being:
> This lets you have the same username as someone else as long as you have different discriminators or different case letters. However, this also means you have to remember a set of 4-digit numbers and account for case sensitivity to connect with your friends.
[1]: https://support.discord.com/hc/en-us/articles/12620128861463...
Not saying that wasn't ONE of the reasons but the main reason was really that a large chunk of users had no idea that they even had a discriminator, as it was added on top of your chosen username. "add me on discord, my username is slashink" didn't work as people expected and caused more confusion than it was solving. This wasn't universally true either, if you come from a platform like Blizzard's Battle.net that has had discriminators since Battlenet 2.0 came out in 2009 it was a natural part of your identity. End of the day there were more users that expected usernames to be unique the way they are today than expected discriminators.
Addressing that tension was the core reason we made this change. We are almost 3 years past this decision ( https://discord.com/blog/usernames ) and I personally think this change was a positive one.
> Starting March 4, 2024, Discord will begin assigning new usernames to users who have not chosen one themselves. If your username still has a discriminator (username*#0000*), Discord will begin assigning you a new, unique username as soon as March 4, 2024. We will try to assign you a unique username that is similar to your current username.
Just some days ago I received warning from Discord that they'll delete my account since I haven't logged in for two years.
> Your Discord account has been inactive for over 2 years, and is scheduled to be deleted on $DATE. But don’t worry! Dust off the cobwebs and prevent your account from being deleted just by logging in.
Imagine trying to connect with your friends... by telephone.