Then maybe you're fighting the wrong enemies. By the sound of it, you're only campaigning against Romans, Greeks, Carthaginians, and cataphracts. Because everyone else takes more casualties from arrows than from bullets or rocks. If you want to defeat Barbarians, Sabaeans or Nomads - or slingers, by the way - you'd better bring some archers.
Not to mention the much higher tactical versatility of archers compared to slingers.
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