T-600's would be easier as they only have rubber skin. The hardest part about the T-800 series would be the living human tissue with blood and hair. T-600 are just steel though and not the mysterious hyper-alloy of the T-800. What kind of metal could resist high velocity impacts?
"The 600 series had rubber skin. We spotted them easy, but these are new. They look human — sweat, bad breath, everything." - Kyle Reese.
Mass produced Chappie would be more likely.
I would agree that drones would be scarier then bipeds, but i think bipedal terminators would still be in existence as they could communicate more effectively and use common tools, vehicles, weapon systems, etc...
It would also be easier for an advanced AI that wants to eliminate humans to just poison us after nuking us and/or use bio weapons. Why didn't Skynet just use gas like Mustard gas or something worse? No real need for an infiltrator IMO.
I'd want an army of IG-11's
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