The actual reason here, implied but not stated outright in that one, is that Discord being a public platform, having only numbers to discriminate between users makes it extra-trivial to impersonate someone else. Obviously you can still do some of this with unique usernames (you see slight misspellings, adding harder-to-see characters like periods, etc, as strategies), but these are more complex to execute on at scale and easier to block once and reduce the impact, vs being able to use ~arbitrarily many post-username numbers.

(i work at discord)

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.