my theory of whats a open world rpg is this

the governing principle is no essential non-player character
a game is truly open world if it does not have any characters that every player always meets

but outside of this pure state, games are more open world the more this is true:
-the fewer characters you always meet
-the less time you spend with them
-the less essential they are

the correlary would be, a truly open world game would have no characters that cannot die (and when a character dies, the game continues without them without breaking anything. a third thing would be, if they can die but dont, they still exist and matter as much as before. the player can choose to kill or save characters and it matters what they choose.)

other correlaries would be, no essential faction. no essential quest. theres no one central faction that every player must join. there's no one central venture that every player must support.
but these follows naturally from the first thing.

the underlying principle is that a true open world game would have each playthrough support a distinct player character. each character makes different choices and does different things. you and your friend play different campaigns. in subsequent runs you play different games.

but morgan brackish meadows, you will say, that doesnt sound very much like any open world game i've ever played. and maybe you will say, i like these games, why should they be otherwise? presumably some people like the grey wardens in dragon age or the dad in fallout 3, some people just play baldurs gate because they want to fuck astarion or something, and that's fine. but its not what we're talking about here.

the answer is, if you want a game that is a movie, that exists. and its done better when its not in the open world mode, probably. the open world mode affords us this other possibility, something to strive for.