AP casters probably wouldn't bust their cooldowns to kill minions anyways. The only champs that use abilities to kill minions are the ones with AOE damage, and they don't need help.
A good example that you already mentioned is AD Malz. While building no SpellPen or AP, his Q,W, and E melt minions insanely fast. Sure, his minions help, but the majority of the damage on minions is from his spells.
The real problem is that Dominion promotes pushing. In SR, if you push too hard, the creep line moves towards your opponents tower. He can last hit easily while you can be susceptible to ganks. In Dominion, ganks are easier to avoid. Also, in SR, a creep wave alone can't take out a tower. In Dominion, two waves can easily neutralize a tower. Of course, auto attackers can push creeps better than nuke casters. AA has no cooldown and uses no resources. Changing the resists on creeps wouldn't really change anything.