The base is very fractured. Even those who play 5.5e don't all agree on a single feel for it. You can't even get anywhere close to every 5.5e player on this site to agree with you about the vibe(subjective feel) of the game or if it needs a dedicated setting, let alone give a single unified definition of what the game is or setting would be. Those disagreements would be magnified as you go out into the player base at large.
There is no definitional agreement like you are claiming exists.
And the "behind the curve" you mention deals with mechanics, not settings which are mechanically agnostic these days.