I can't answer why GPUs weren't considered, though I'd guess because they weren't as familiar with the numbers and this was a "back of the envelope" calculation

I can't answer why GPUs weren't considered, though I'd guess because they weren't as familiar with the numbers and this was a "back of the envelope" calculation. The reason why I wouldn't take into account better attacks is because that's much harder to predict than Moore's law. Also, this is supposed to be more of a worst case scenario, so even if no advances are found it will cost at least that much.