The number of offensive slots doesn't matter as much as the size of your offensive allies. You could reduce it to one slot and still have the same disadvantage if the attackers are sufficiently organized.

With even sized allies, it balances out attacking all-turret defenses. Taking express as an example, 3 pure defense commies are def allies and have 1m turrets each. The break is 1.5m SS, so an attacker need 500k jets with 3 allies with 670k jets, meaning you'd still need to be >50% offense/defense split to break them.

If any country became unbreakable, the game would be a runaway.