There is an ongoing cost to holding it. Your wealth is secured by the network, without that security, your wealth is worthless. That security you get is subsidized by spenders, and this has incentive/game-theoretical implications for the bitcoin network that are not good. You can have the rarest of assets and if it can disappear while you sleep it's not worth anything.

Energy becoming cheaper... As energy becomes cheaper, people spend more on it by raising their energy usage. This is commonly observed everywhere in the world, and so far, empirically, this has held true for the bitcoin network as well.

I explained all of this stuff the other day in more detail in the following note, as well as another note in the same thread, if you want to give it a read to get a good idea of my thoughts on the matter.