This is THE thing people overlook. There used to be trees there! There used to be lots of vegetation, that's why there's so much carbon trapped in the permafrost in the first place. We're not releasing something bad, we're returning the earth to a better equilibrium. Imagine if the Siberian tundra were replaced with forests and farms. Imagine if we avoid the next glaciation - which is coming if we do as the climate alarmists want, yet there is evidence that we can't prevent it even if we try.
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.
And remember, the alternative is still glaciation.
The alternative is glaciation? If we have enough control over the climate to make it a bunch warmer, surely we have enough control (should we choose to exercise it) to keep it in the middle.