Sure. I don't believe that the marginal cost of anything can be close to zero without there being an infinite demand for it. You mentioned that if the marginal cost of energy gets close to zero, then there'll be no incentive to save energy if energy costs virtually nothing. Demand will meet supply and there'll be a cost to it, so its cost won't go back to zero.
You may think that streaming costs zero, that the marginal cost is zero, but eventually when everybody is on the Internet, as we are finding out these days in our neighbourhoods, Internet starts to be a bit slow and Internet service providers start thinking about increasing their prices because they'll have to increase the bandwidth.
So I don't think that it's pointless to fight deflation, because if the marginal cost of something goes down to zero there will be an infinite demand for it and it will mean that its marginal cost will not go down to zero. There are, of course, exceptions, but generally speaking, I don't think you can have deflation that's unbeatable or “unfightable”.