The reality is that there are almost ten billion people on this planet and they live for upwards of a century. You are simply deluding yourself if you think it is reasonable to build a system with unique, permanent usernames. Nothing in the real world works like that, including trademarks. And it just helps enforce the very problem that people try to trust usernames and then get tricked by people who sniped usernames that are tied to other peoples' well-known identities (leading to abused "verified" badge systems and legal challenges and expensive hostage scenarios... it just sucks).
And for what? To make it easier to hand-type a URL? Does anyone even do that? I am super technical and I barely even do that in 2018, as if nothing else there are too many websites in existence to remember all of their one-off URL schemes. Like almost everyone, I either use the site's built-in search feature or I do a search on Google to find people, and let a combination of page rank and personalized results guide me to the right destination. Some web browsers don't even show URLs anymore!
Here is a great example of where it is completely insane: Facebook. There is absolutely no good reason for that website to have usernames for regular users, and they frankly shouldn't have usernames for businesses either. It isn't even clear to me that the app--which most users are using, not the website--even has a way to show people's usernames, which means this is an identifier which somehow everyone knows must be chosen and must be unique and is nigh-unto permanent but which somehow is also simultaneously meaningless but is also a horrible point of contention? What?
I am lucky. I spent a bunch of time in 1994 to select a username, and despite being 13, I was mature enough to come up with something that wouldn't ever come to cause me complex problems. People ask me what it means, and it essentially doesn't mean anything: it has only a positive connotation to me when I hear it, it is entirely neutral, and it had no existing usage I could find. Yet, I also still got screwed, as I am semi-famous, and everyone knows me as this username. I have kids who look up to me enough to want to take my name as a show of support and I have to essentially be the big bad asshole about it because in a world of unique and permanent usernames, people then assume the kid is really me. On the other side, I have been asked to rename myself by moderators of various forums as they couldn't believe the real saurik got an account on their site, and it was "confusing" people.
And so in the end we all have to deal with the worst-case scenario anyway: unless you do nothing but sign up for random sites rumored to be interesting constantly (which I seriously tried to do), you eventually will succumb to needing a way to prove who you are on multiple sites and tie together those identifies. And for most users... as in virtually all "normal users", that moment comes when they are using only two websites, as their username was probably something like jay.freeman.178 as everything that was even remotely interesting to them was taken a decade earlier by literally a different generation of humans, so they let the website automatically generate one.
In a world where everyone is having to solve the worst-case problem anyway, every site should just have numbers as unique identifiers, at most have some kind of trust score for degrees of separation on the site (so you can get a feeling for "is this the saurik that I met?"), and everyone should be trained "names don't matter and if you see someone with that name it doesn't even slightly mean that they are the same person you met last week".