-Power plants require humans
-mining/processing/manufacturing still requires humans
-There's not enough automation for artificial intelligence to take over. T-X's ability would be lost with her destruction, too. Can't tap that.
-nukes would damage/destroy much of the world's industrial infrastructure
Flying drones and robo-tanks can't do everything. Versatile humanoid robots don't exist and to create them would require a lot of work/development. Seems to me like the window of opportunity would be a large enough one to shut down every resource Skynet has before it rolls out enough terminators to seriously threaten humanity.
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