Unique usernames cause issues with having to come up with wildly "creative" names to be able to be called the same thing everywhere if the platforms get big enough, or start adding loads of numbers to everything. Unique usernames also create prestigious names, which people most certainly care about, especially for branding or clout.

The discriminator system was clever, as it let people just pick names that they felt resonated with them better, while also keeping everyone equal - you didn't have a prestigious name like "adam", and everyone else who just wanted to go by adam on the internet had to add extras to it. Instead, everyone was given the extra numbers so nobody was special.

Now obviously, the username in itself is not that special, and nobody really goes by their username, but a lot of people do assign some value to their online identities and handles, and Discord just sidestepped that by putting everyone on the same level.

There were issues with the username system, but I don't think the 4 numbers were it. You absolutely do memorise your 4 numbers if you're adding people constantly for some reason, and if you're not, it's not exactly a major inconvenience.

From my experience, people who are happy with the new system already go by somewhat unique names, so this makes it easier for them.