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All it would take would be for photosynthesis to be terminated, and then wait a few million years for erosion and volcanism to expose enough reduced material to soak up the atmosphere's oxygen.

What's weird is that, as far as I know, there's no feedback mechanism that's been identified that keeps the atmosphere's O2 level stable. It may have been stable since the Cambrian just because if it hadn't been, we wouldn't have evolved, an anthropic argument.

Fire must surely have a role to play there? Too much O2 and plants burn easily, too little and fires won't take hold.

And we don't know AFAIK that it's been entirely stable. There's some debate over what the level was in the Cretaceous for example.

If anything, fire would be a positive feedback. That's because fire produces charcoal, and charcoal doesn't decompose. Instead, it gets washed into the ocean and eventually buried. It's not photosynthesis itself that causes O2 accumulation in the atmosphere, it's the burial and sequestration of reduced material from photosynthesis.

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