They don't make those M1s like they used to. Plus, those spearheads are made of depleted uranium.
Actually, if you browse civfanatics.com
you might be able to find something in their mod database that does this.
If not, and this really
matters to you, you can always use the editor included with the game to set up a map where the rules have been changed. Give the modern units a 300% increase in attack-defense strength or something. I think Modern Armor units are 24-12-3 (attack-defense-movement)? If you change it to 100-50-6 then little 1-2-1 Spearman will die easily, DPU spearheads or not. You can also change their default hitpoints. I think all units have three green health bars. Let tanks and the like start out with four or five, just to make certain some dude with a musket doesn't pop the turret off your Abrams.
The big problem with this solution is you must do this every time you want to play with these rules. You can import rules from one map to the next in the editor, and the editor itself can generate a random map not unlike the game can, but you can do none of this in-game.