Quite simply the www subdomain is confusing and unnecessary. See comment from the ISP admin I cited re: user training.
> Maybe we should be hiding ".com" because that's trivial too? Better yet, why show "google" at all if from the page it's clear you're on Google?
Those aren't really serious counterexamples. ".com" is obviously not trivial; there are plenty of TLD variants in use. It is approximately 0.0000000001% as common to run separate HTTP servers on example.com and www.example.com. And clearly different domains can spoof one another's content so that's not a way to be "clear" you're on google.com, whereas this is not generally an issue with subdomains.
Again your reaction is just sort of knee-jerk exaggerated resistance to change, not actual real world problem cases.
Sometimes it's unnecessary. How is it confusing? Millions of non-sophisticated users became sophisticated users typing it, millions more type it every day. It doesn't seem prima facie more confusing than a pronoun or other oft-repeated article. Consider the beginning of my last sentence in this paragraph -- would you consider the "It" confusing, even though it's not strictly necessary?
> Again your reaction is just sort of knee-jerk exaggerated resistance to change
Perhaps your reaction is knee-jerk teleology of change as progress?
As a suggestion: maybe spend less time characterizing the approach of people that you disagree with on this topic, and more time articulating actual arguments ("the www subdomain is confusing and unnecessary" counts, even though it's arguably not particularly strong), unless you'd eventually prefer it when people make the discussion partly about the shortcomings of your approach, which are far more glaring than you've clearly spent time considering.
mail goes to mail.example.com
voip goes to voip.example.com
vpn goes to vpn.example.com
Why should www be any different?