Hank Green put out a rather interesting video about Jevon's Paradox - "Unfortunately, You Need to Know What the Jevons Paradox is".
The basic idea refers specifically to coal, but extends to really, any recourse using process:
Technological improvements that increase the efficiency of a resource's use lead to a rise, rather than a fall, in total consumption of that resource. - Sorrell, Steve (2009). "Exploring Jevons' Paradox". Energy Efficiency and Sustainable Consumption: The Rebound Effect. London, England: Palgrave Macmillan. pp. 136–164. ISBN 978-0-230-58310-8.
If this principle applies to the cost of coal (and if it mostly proves true), it could extend not only to the cost of AI use, but the environmental impact as well.
If we improve AI to the point that the impact of its use drops drastically, but we then become less uncomfortable with that impact (it's better than it was...), then we'll likely use it more, and if everyone uses it more, there's a good chance that will offset the drop in environmental impact and be worst for the environment.
Though this is more of a hypothesis and less of a proof, it stands to be a major concern we need to consider regarding technological advancement.