Maybe Zelretch is fine with the FGO timeline going to shit, ever considered that
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If you really want a rationalisation as to why he wouldn't get involved, look at the single scene where we see him observe worlds using the Second. Meddling directly runs the risk of confirming a timeline according to Nasu's creative interpretation of the observer effect, while FGO seems to be resolving itself just fine through (your) heroic actions.