I think if the play is far enough away then it should be just play on, especially if the player coming on has no involvement in the play. That can be decided by the bloke down on the boundary who puts the flag up for the 50m penalty, just show some common sense, if the play is say within 25 mts or so or the player runs on and makes a tackle or is passed the ball then pay a free kick, no 50m penalty, just a free kick.
I agree with most of Romper's points except, if the player coming on has an immediate impact, because he was within 25mts of the play, then his Team has received a Tactical Advantage though him entering the field of play at that time. In that case, I would apply the 50 metre penalty from where the player is standing.
No demetrious and co. are too stubborn
I think that if they go past the "gate" line but are still in the out of bounds area, it should just be a free kick where the ball is at. Only apply the 50 if they enter the playing field before the other player is off.