Instagram usernames are much easier to reason about if you don't understand/care about the differences between domains, subdomains, tlds, protocols etc.

We gave people a messy solution that assumed they would care about implementation details.

I don’t know if domain names is really that much better a solution—between the messiness of protocols, subdomains, and TLDs (how many kids in school went to whitehouse.com instead of whitehouse.gov back when the former was a porn site?), they can also expire and be replaced with a domain showing very different content. Lidl recently had to recall some kids snacks because the URL was not showing what they wanted it to.