If Bitcoin was that big, there would be savings from not having the other activities and businesses that are polluting right now.
You have to compare the cost with the benefit. If the benefit is huge, the cost can be very big.
In the hypothetical situation where bitcoin replaced most of fiat, I don't see what business would be not there anymore, except a certain part of current banking (probably replaced by similar bitcoin financial services). Bitcoin would then simply the means of payment, like fiat is today. However, the mining logic of bitcoin is such that there's an incentive to invest more hardware and energy in bitcoin, until there is an almost break-even between the mining seigniorage and its cost. The mining seigniorage is the bitcoin inflation which is now about 10%. It would hence mean that of the order of 10% of the $ 50 trillion market cap would go into mining hardware and energy consumption per year until mid 2016. After that, it will be 5%. That's still $ 2.5 trillion worth of hardware and energy cost.
No other pulluting business would be replaced and out of business with that (except of course that it wouldn't get any hardware and any energy anymore as all of it will be used up by bitcoin miners).
There is no particular benefit (on the pollution/business side) of using bitcoin over fiat, that would reduce in the same way the mining pollution and consumption. Even the total banking world today is not consuming all available energy and all available hardware.
