This rule could never work, too many things can happen that would make trying to police it a farce.
What if you were one of the final 3 and your lead and mark ends 2 metres outside the 50? Or the ball was bouncing around just outside the arch, you'd have to wait there until someone fetched it for you.
I suppose you could be allowed to leave the arch once the ball is kicked, kind've like the offside rule, but again, too hard to do, umpires get enough [censored] wrong, imagine them trying to get that right.
There are also injuries and interchanges that happen, you'd have the AFL's little post-it notes on the field saying who is allowed to exit the forward 50 because they need to get off the ground.
Impossible.
When teams start losing matches because they're streaming down the ground but there's no one to kick to, the game will fix itself.