>Think of where batteries have gone in the last 20 years. Where will they be in 30 years from now?

Is this actually a reliable trend though? We're talking about charge density (energy per unit mass) specifically. Lithium battery tech was invented in the 70's and developed significant charge density refinements up through the 90's. (This is based on a bit of research and my historical recollection; I was unable to find a reliable chart of lithium battery tech charge density over time.) And although there have certainly been further improvements over the last 20 years, my understanding is that they have been incremental improvements and with diminishing returns. Although we may still be able to squeeze out a few small improvements, lithium battery storage tech has already been developed to store close to its maximum theoretical charge density.

Additionally, lithium battery tech is close to the best theoretical electrochemical scenario, so there isn't some new chemical battery tech "waiting in the wings".

At this point, further significant advances in battery charge density will likely require a fundamental breakthrough; and these are uncommon and unpredictable.