They are exactly the same kind of error in that both assume stability over time. One assume prices are stable; the other assumes the rate of change is stable.
But the issue is that, _even at the time of the statement_, the price has been decreasing over time! It's falsifiable at the time of the statement! You don't need to see the future to dismantle that argument.
Inversely, costs have gone down over time for a long time. You could make the inference that there's a floor, of course, and it's reasonable to do so! But it's hard to disprove the claim that prices will keep on going down.
The former is just on its face wrong based on the current facts, the latter is a judgement call about the future. Totally different beasts, and driven from different things.
I agree that it's generally more likely that a 15-year trend will continue than change. If we're talking about a year, that is. But 5 years? 15 years? 100 years? 1000 years? At some point, the general assumption changes.
But without trend data, I also agree that assuming price stability a better general assumption than assuming a major price drop. Historically, very few things keep getting cheaper. It requires a) large society willing to keep making R&D investments, and b) a technological domain with a lot of possible ways to keep lowering costs.
And what I mostly agree with is the proverb, "It is difficult to make predictions, especially about the future."
At some point, there's going to be a price floor that the research, materials supply and competition simply won't break through.
Guessing when that is going to happen is more luck than anything. I do not see it continuing to get exponentially cheaper for long, though.
LiFePho has removed most of the precious metals out of the equation, and the demand for electric cars will continue to compete against the growth in demand for battery storage for renewables. For an analogy, lumber prices have shot through the roof over the past few years where I live due to construction booms. Nothing about the technology has changed, and supply hasn't fluctuated greatly. These same pressures are going to be pushing against lithium batteries getting exponentially cheaper over the next few years. I don't doubt that they will find room to bring prices down, but there is a floor out there somewhere close by.
Electric airplanes could carry these for takeoff and maybe jettison at a preplanned location for recycling.
Regardless, 1 year isn't the correct duration for a bet. People average owning a car for ~6 years, and the average lifespan is something like 12 years.
But depending on terms, I might take a year-over-year bet for battery prices. Demand is high and the pandemic has caused significant supply chain problems. They could well have gone up this year. And indeed, a quick look at news reports suggests key components, including lithium and cobalt, are surging in price. Fine examples of why assuming a historical average has future meaning can get you into trouble.
Lithium prices have gone down in the last few years, and Cobalt isn’t a necessary component.
Meanwhile LiFePO4 batteries, which are currently the most popular chemistry(at least in China), contain no cobalt whatsoever.