I think the main defensive part of the game at present the AFL is trying to address is the ability of 10 or more players from each team to charge manically from contest to contest, and then resting after each 5 or 6 minute ballistic burst.
This creates the blocked up, high stoppages, 'rolling maul' effect that stinks up so many games.
I generally concur with the broad concept that the game is less attractive when the midfielders/runners are able to rest up every few minutes. When unlimited interchange was first introduced in the late 70s/early 80s, I am sure the rulemakers of the time had little idea what it would lead to and would've thought twice had they have known.
Whether these current changes result in the desired outcomes remains to be seen, no doubt there will be unforseen consequences.
On face value, I'm happy to give it a chance as a step in the right direction.