They made a mistake. The four on the interchange was never intended to be used in the way it is now.
The game evolves.
It doesnt mean it needs rule changes every time a coach innovates.
Every rule change will result in new tactics. All the AFL is doing these days is brining in rule changes as a response to coaching strategies which often have occurred as a result of previous rule changes.
Bottom line as I said....the game is still 18 v 18 regardless of the interchange rules.