Shortly after, they started hiding www, and openly said that they intended to eventually hide the amp prefix as well - which would help their own goals.
"www." was never a mistake that needed to be hidden, any more than any other subdomain. I bet that if Firefox had 90% market share and decided to hide the "translate" or the "maps" of maps.google.com, that Google would not find any justification amusing. www is a subdomain, and if a dev doesn't think it should be there, he could just avoid using it.
It opens security issues (by showing the "wrong" URL), usability issues ("read me the URL letter by letter"), and it forces developers to deal with a incorrect setup that can change any day. What if Google then hide "app." or "dev."?
But Google does what is best for Google.