I've heard it's mostly a fallacy. Jevon's paradox is the assertion that demand for energy is so elastic that increases in efficiency increase demand so much that this overwhelms the savings from efficiency. And in most cases this isn't true. One must be careful when looking at historical evidence to not say "efficiency increased, and demand increased, therefore demand increased because of efficiency", which is just a post hoc non sequitur.
I guess your point is that as energy gets cheaper demand will increase therefore negating the savings. Well, that has not been the case when it comes to power. Look at the cost over the last 200 years and you'll see that the cost has dropped over that time. Energy cost is a fraction of what it was then but demand has exploded. There's no reason to think that things will be different in the future.