Imagine if for this movie they just ignored the whole dumb "evil John" thing and the main bad guy really was the Asian T-1000. They defeat the original T-800, they defeat the T-1000, but then Pops explains to them that while the war technically "ends" tonight with this being the final victory over the machines they still can't change the timeline too dramatically or they will actually undo events that *did* lead to their ultimate victory. So Pops instructs them to take the original T-800 carcass to the Cyberdyne factory so it can be found and then later destroyed by Sarah and her 10 year old son.
Reese and Sarah comply. Then Pops says, "there's still one more CPU, and it must be destroyed as well." Everyone's thinking oh no now he's going to sacrifice himself again. He casually reaches into his jacket, pulls out a Desert Eagle...and blows Reese away right in front of Sarah! Sarah, (who's already pregnant) freaks out and somehow gets away (maybe Pops' gun jams or some other fluke allows her a bit of a head start) and Sarah, alone, while grieving the loss of her lover (AGAIN) must fight not half of a T-800 in a hydraulic press but a walking fully functioning killer robot. Without Reese.
Bring the whole thing truly full circle. Because what Skynet determined was that they MUST be born in 1984, which means they can't go back to the 70's to kill her, or to the 1800's to kill her great great great grandmother or whatever, they have to make sure that events play out right UP to the T-800 being found at Cyberdyne, THEN Sarah/unborn John need to die. So they sent "Pops" as the ultimate infiltrator, a contingency to stack the deck in their favor against Reese when both he and the first Terminator arrive. Pops' mission was simply to make sure the first Terminator goes to Cyberdyne and...with the full trust (instead of terror) of both Reese and Sarah that would be when he'd stop speaking (permanently!) and simply systematically kill them both.
Reese's head gets blown off in horrifying fashion and Sarah is forced to have one final showdown with her ultimate enemy, but this time as a seasoned warrior herself.
