Sandbagging is actually going to get worse the stronger defenses become. What side would you like to be on?
15 CWA and 15 Iron Or 5 CWA, 5AA, 5GA, 5IA, 5CA, 5GPA, 5 Iron.
On one side there are 5 very hard bases to 5*, on the other 15. Sure with sandbags you are giving away 15 5* attacks, but you are getting the ability to get pretty easy 5* attacks on 20-25 bases in exchange.
Meanwhile the defensive buffs hurt the middle players more and more and more as they cannot attack up and their targets are increasingly bags.
Even if you get max defense as a middle level player you will always be facing someone who can 5* you. If the sides are perfectly even then the higher ups on each side roll the lower age players to get the easy 5* and then it's who can afford more EA to get the 5* for the win. No amount of defensive buffs that are remotely reasonable will allow lower level ages to defend against higher level ages. So the answer is more bags so you can have more of the higher level age accounts in war than the other guy.