You're making the assumption that users know YYYY-MM-DD is the date format and not YYYY-DD-MM. I bet if you asked non-developers the answers you would get would be pretty random, probably more errors than the locale on their machine.
Wikipedia claims [0] that is the date format in Kazakhstan, but only for the Kazakh language, not for Russian. (Maybe there is someone here from Kazakhstan who can confirm if that is true or not.)
A lot of companies don't have any employees in Kazakhstan (which means they can ignore this for internal systems); and many companies wouldn't have any customers there either (which means they can ignore it for external systems as well).
