Recently overheard:
"<RFDS call sign>: You used to always fly this segment on FL160, but recently it's been FL180, why's that?"
"Saving fuel, it's all about the carbon foot print".
And that describes the cold hard truth: most operators have a lot of inefficiencies, simply because nobody has properly thought about the savings to be had.
Yes, they could find those savings without a tax, but they weren't. Hopefully a tax or ETS will inspire more to do so.