I still think that your usage of "0-day" breaks down precisely in the case we're in currently, where the vulnerability has been exploited in the wild and Microsoft has known about it for some time, but there is not a patch available, and the general public (everyone who has to defend against the exploit) found out about it today.

I’m a fan of letting context infer meaning. And letting certain words just naturally grow to whatever the culture wants it to. It’s always hard to fight back against it.

There’s a million examples of this on the internet where people try to be pedantic about slang or word usage. All that matters is “we know what you mean”. I like to assume enough people here know the real difference zero days vs existing vulnerabilities are. But in practice it matters less.