It is endlessly frustrating that companies have universally decided that they won’t let people say “no” to stuff, ever. There are no longer options to reject stupid-ass new “features”, only postponement until next time you open the app/website/program. They’ll continue pestering you for the rest of your life. I realize that my frustration may be a little over-zealous, but we deal with these interfaces dozens of times per day and this is user hostile behavior. There isn’t really an option to just use another service or program, since the entire technology landscape has been commandeered by a few major corporations, and they all enact the same shitty things as a group.
Discord wants to move away from the #4-digit identifier system because it is an unnecessary hassle for users. The downside is that it means that a username must now be unique. That’s why they’re asking you.
I guess at one point they’ll just force change your username and you’ll have to stick with it or make a new acc.