I feel like icq got this right, no one had a name, just an identifier of n digits.
If we all used some central service that provides ids, user profiles could be linked between services.
It was a terrible system, the worst, really.
Usernames aren't a problem, discoverability is. Vanity IDs aren't a problem as well, but an opportunity and they are being sold either for money (registered trademarks, "lucky" phone numbers, short or custom license plates) or engagement (Facebook's vanity URLs for pages).
Vanity id shouldn't affect discoverability and it really doesn't these days. Do Nimble CRM's customers have confusion by @nimble on Pinterest being taken by the Author? Don't think so. pornhub.com ha more traffic than sex.com and doesn't seem to be bothered.