Yeah, but you're forgetting, they're from the future so it WOULD be an instant loss, to them.
But how would it be any loss, instant or otherwise, if the T800 never finds her? Mind you, I still think he WOULD have found her even without Reese's intervention, but you did put forth the theory that he NEVER would have found her without Reese.
So . . . under the "No Reese, no findy Sarah Connor" theory, they send the T800 before Reese, the T800 fails to find Sarah, and the resistance doesn't lose, instantly or otherwise. Then they send Reese, so the T800 finds her, but still he fails as the events of the film unfold.
Mind you, I don't really believe the theory, because I think the T800 would have found her anyway, eventually. It's what he does. It's all he does. You can't stop him. He'll wade through you, and . . .