Interesting point. But really it seems to me you're saying the slaves are the logistical support, not the fighting arm.
We already have robots doing logistical support.
Slaves were not the logistical support but the cornerstone of their entire economy.
States like ancient Rome/Sparta or more recently southern US states pre-civil war have become utterly dependent on slave labor and were unable to let it go.
And to get back on topic: