People will certainly use more energy as it becomes cheaper, but not enough to stop it from becoming cheaper.

I suspect you're right. As long as we're drawing electricity from an electric grid, which obviously has to be maintained, there will be charges for usage. However, the goal is to charge customers just above their marginal cost (assuming a bunch of stuff about competitive markets).

Marginal cost might become only trivially based on usage, if energy becomes cheap enough and instead be based on the costs of maintaining the grid. That's how I could see energy becoming more of a club good, where you just pay for access to the grid (and maybe for some max usage rate).