Every club knows how to defend the ground and how effective it is. Taking out players may make it more difficult, but definitely not impossible.Because fewer players on the field makes defensive zones impossible to implement. A player with the ball can kick the footy 50m in a a second or two. If you have players set up in a zone in the field ahead of the player with the ball fewer players will stretch the zone meaning the player with the footy can just pick their way through. The defensive players will not be able to cover the ground in between them in the time it takes the ball to get from the kicker to his teammate. It will force teams to go more man on man.
No doubt there will still be instances of teams "flooding" but for the most part general play will open up and get back to more one on one contests that we grew up loving (Jakovich/Carey, Silvagni/Ablett, Knights/Van Der Haar etc)
Look at it from a coach’s perspective
The game gets cut to 15 a side. Your 18 man defence is now broken. But you know that a system like that is going to work, you just need to dedicate the time to adapt it to the new conditions. If you don’t do this, every other team that does will have a distinct advantage over you.
Another thing you can do is develop a method to break down opposition defence. This will be a great asset if it comes off. But that’s not guaranteed, your method cannot be fully proven until it is tested in game.
It is clearly the best option to fix the defence first, and then use whatever time is left to develop new methods.
So we end up with almost the same game again the next season, and the cycle repeats - another poorly thought out rule is brought in, defensive structures are fixed, very little development on attack. If we’re lucky, the rule will have a laughable loophole that teams will exploit.
The one thing that can be done to ensure the game stays in its current state, is to continue tinkering with rules every year.