I agree with the spirit of your suggestion that we should remove clear instances where the law says do A and everyone does B, particularly where players, coaches, spectators, pundits and officials all prefer and expect B.
Your example highlights why it is not always that simple. As others have said, I think the intention behind the original law was to deal with the wrong number of players being on the pitch, not encroachment, and I think the change has to be to clearly separate the two. Otherwise a team can have 12 players and the goal stands as long as the additional person is not involved in the goal - and by the way if it is a substitution gone wrong, is the player who didn't leave or the player who came on, the additional person ? And if it was a game with no subs cards (everything below Step 1 in England), and you had a 3 on , 3 off scenario, even if you decide substitution not completed means the player who came on is the additional player, how do you then work out which of the 3 who came on is the additional one ?
That also illustrates the challenge of creating a set of laws that is so long and intricate that it becomes counterproductive - further up the thread I gave the example of the offence outside the field of play when the ball is in play section. It is now complicated enough that it is at the stage where I am pretty confident in saying the vast majority of referees don't know it - I reread the Laws reasonably regularly, and each time I do I have to pause and read that section several times to see through the fog. I'd like to see fewer examples of that, not more
