There is a field for the full name of the person if you want to, and at least on my linux it warns for non-ascii characters but allows them
You started off with saying that there's no value to login names beyond ASCII, and now you're (a) insinuating that anyone who would wish to have login names beyond ASCII are too heavily invested in their login name, and (b) there's technical downsides to this.
Point b is certainly valid, and the whole reason there's a discussion in the first place. Point a might be true, but very different from what you originally wrote. And again, I suppose, this doesn't apply to you, because your login name – which you isn't attached to at all – is naturally ASCII safe?
What I mean is that it's costly and the benefits are at best emotional.
And if you personally assign a large value to having unicode characters in your login name specifically, I posit that you have the wrong priorities in life. You're free to disagree with that opinion