The key thing that is missing is that while battery capacity has increased incrementally the price has dropped by a good amount every year. The last figure I saw a few years ago is that the price has dropped 10% every year since 2013.
Even if the capacity never goes up 10% with current technology, battery powered cars will get cheaper and cheaper until they are on par with ICE and then fall below it.
Past performance is not indicative of future results.
I would have to understand why pricing has been on that trend. I have some theories, but I'm not close enough to that market to know.
What I do know is this:
Prices can never, long term, decline faster than production costs.
So pricing essentially will be based on the following, long term: cost of inputs + manufacturing/distribution costs + profit.
But there are other things which affect this. There's something known as Wright's Law
Wright's Law which basically states that as volumes increase, costs decrease. This is due to a lot of things, but battery production volumes have increased SIGNIFICANTLY over the last decade. Wright's law may suggest that operational efficiencies, supply chain efficiencies, fixed costs amortization across larger volumes, etc all improved to reach that 10% annual reduction. There could of course be reductions due to technology improvements allowing use of less costly anode metals or other aspects that brought it down as well.
However I would argue that Wright's Law reaches a steady-state or point of diminishing returns once volumes reach certain points, because at some point you've teased out the operational efficiencies and it just comes down to cost of inputs. So at some point battery cost declines might slow if they were driven by things like Wright's Law more than they were driven by inherent improvements in battery chemistry (i.e. storage capacity gains).
Which is a long way of saying that I'm not sure we can predict a solid 10% annual price decline for the next decade just because we saw it in the last decade. The market was going through a rapid volume increase and market maturation period, and eventually all of the low hanging fruit will already be picked.