Even if everything is HTTPS as you claim (spoiler, it is not, there's still plenty of other protocols in use today) subdomains are still relevant information used in SNI to serve virtualhosts.
There is no reason that imposes www.example.net to serve the same content as example.net. Whether or not it does is left to decide by the website operator.
You're right that it isn't everything, but the non-https stuff out there is super buggy. I had a Canon 5D Mark 4 and it had the option to (s)ftp up raws so I wouldn't have to take out the card. So I went to the trouble of setting up my own secure ftp server. Guess what. It didn't work. I tried everything from raw IP, to a naked domain, to a ftp domain, whatever it was there was absolutely no way of getting Canon to make the connection securely.
That's why everything is moving to HTTPS endpoints. It's simple. You just give a url and it works.
Specifically SFTP is a SSH subsystem, when your client connects to a SSH server it gets to specify what subsystem it intends to talk to, and so a remote SSH server can choose to offer the "traditional" shell service, an SFTP server, or any arbitrary thing. In this way SSH subsystems are rather like ALPN in TLS (except years earlier) which is how HTTP/2 works among other things.
There is no reason that imposes www.example.net to serve the same content as example.net. Whether or not it does is left to decide by the website operator.