1. you never know when you need to write https://
2. it looks a lot uglier
3. literally means "hypertext transfer protocol (secure)" (I had to look it up, and i am a hacker news user.)
4. just happened to me: in a youtube description you cant just put ycombinator.com, you have to write https://ycombinator.com. This is not a problem if We Write World Wide Web When We Want to link to a Website
The problem is your examples assume you're talking about ONLY a web server, when you can put ftp:// or ssh:// or even gopher:// in front of ycombinator.com and the connection process will be vastly different, as they are different schemes of communication. Websites will always require a valid protocol before the domain because otherwise there's no way to know for sure where the link is meant to go.
It is not a proper URL without the protocol part up front. Browser creators improperly trying to hide the protocol not-withstanding.
> just happened to me: in a youtube description you cant just put ycombinator.com,
ycombinator.com is not technically a URL, it is simply a domain name. The fact that browsers all invisibly 'auto convert' it to a URL when entered into their URL/magic bars does not magically make it into a url. If youtube's code which is looking for URL's to automatically make them anchor links simply looked for words with dots, then a large amount of false positives would occur (things that are not URL's getting wrapped in anchor links).
You can blame the browser makers, and the evil one specifically, for all the hiding of URL structure such that people now think "ycombinator.com" should be interpreted as if it were a URL everywhere.
www.news.ycombinator.com
www just means "website"
I don't think you understand what a URL is, it's more than just the address. Ftp://ftp.yourdomain.com and many others, for example.
However you do raise a worthwhile point in that browsers are overfitted for HTML when they were originally intended to be universal network interfaces.