>There's no real reason to do it, other than convention from the early days. //

It wasn't convention it was that the naked domain wasn't assumed to have web content as not all domains had web servers on them. If you're used to gophering in to balrog.example.com then a www probably was required to get a browser to reach the www pages.

Supposing VR takes off and people have a VR space as their primary internet presence then we might be having the conversation as to why do we both with the vr. subdomain on all the URLs.