Hey, they can take Sakura and make a happy family together.
Well, her past and her reason for becoming a CG makes quite a bit of difference. If, for instance, she knew about Sakura's plight, then she would most definitely want to help her.
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I don't think so, actually. Assassin is good at taking down masters, but not servants. Kiritsugu would still need to find a way to draw them out into the open, away from their servant.
Why does he have to fail? He has several people who would gladly help him.