There's over half a million images with this tag, so maybe it's not a decision that should be taken lightly? The whole value of tagging images is that they are tagged permanently. If years worth of tagging gets nuked over technicalities like "none of these teeth are prominent enough" then what is the point of tagging anything in first place? imo teeth is a very obvious tag to use and any tagger can tell if there is teeth or not in an image. Plus upper_teeth_only is like 40% of teeth, so I feel like you could use the same reason to get rid of that tag and then similar tags until most of the more common tags are gone just because almost every image has them. Like there's 4 million images tagged breasts and they don't have prominent breasts. That's what breast_focus is for.
For prominent teeth you could just search for teeth mouth_focus. You don't need a new tag for this.
I doubt anyone is actually searching for teeth on booru, btw. If they ever use this tag, it will be combined with others like drinking_straw teeth.