Imagine if the Siberian tundra were replaced with forests and farms, and current forests and farms were replaced with desert.

You can't just look at the potential upside, assume it will be huge, and also assume there aren't any negative effects that might outweigh it.

The argument that the Earth used to be warmer or cooler or whatever is completely nonsensical. We're not worried about Earth, we're worried about people, and more specifically civilization. The fact that the tundra used to be lush doesn't mean anything in that respect.