It doesn't really matter. If the player is standing beyond the baseline when they are hit then the ball had no chance of landing in.
People are trying to suggest that these are small margins when they really aren't. If a player is standing outside of the court when the ball hits them then the ball has already passed beyond the point where it could possibly land in the court.
The exceptions to this rule would be so rare as to be inconsequential. A simple rule stating that if a ball hits a player who is standing outside the court (or service box for a serve) then it is called out would have no practical downsides.
And no, this doesn't happen very often, but there's no harm in changing the rules to get it right when it does.
The only distinction you'd need to draw is the obvious one, that if a player plays the ball with their racquet then the ball remains in play. I suppose you could then have an argument over whether a player played the ball or, as in Cornet's case, was taking evasive action and the ball simply hit her racquet but I'd imagine it would pretty obvious in most cases and easy for the umpire to decide.
You are correct that it happens quite rarely, but we don't want to change the rules so that it encourages any increase in confusion or its exploitation. Ie, it could encourage players to be more willing to stand near the center service line against a good server so that in case they get hit, they can make an argument that the serve should be a fault... or even more likely, to distract the server.
In sports it's generally better to distribute discretion to nearly inarguable realities, not place it in the hands of a single human being who may not notice certain things happening or interpret something differently. If you must have a rule change that leaves it up to the discretion of one sole person, it should be specific enough to only apply to doubles, in cases where the direction of the ball is clearly and inarguably
heading to the wrong side of the court, and where the "hit" player is nowhere near the center service line. The last part is where it gets a bit hair-brainy and you would need to find some way of specifying it or simply leaving it up to the discretion of the umpire, which isn't a great idea (but for something this infrequent at this point who cares). Another way to insulate the rule is to make it so that it initiates a replay rather than it counting as a fault or point (to the "hit" player team). That way, it's not worthwhile to exploit AND more importantly, both teams have responsibility and neither are at fault, which in reality, best reflects what actually is the intent of the players when something like this happen.