There's 2 big problems with this.
- Light is what is being purchased, not power.
- The word "merely".
If I understand right, decreased cost of light (not power) will result in an increased use of light, for sure. However, the details of the light demand curve, and the details of the cost structure of the light will determine net power usage devoted to light.
While the demand for light will almost certainly go up, so too will the power usage of the light we're currently using go down. It's not hard to picture a world in which the (substantial) cost savings (97%?) on electricity from good LED lighting mean that our existing lights save 97% of their power cost, and we then only use double what we were using, thus saving 94% of power usage on lighting. Perhaps that edges up over time. But the "merely" is highly deceiving.