What you are saying doesn't even make sense. The original comment was that a battery that lasts 5x as long costs 1/5th the price over time, and your hypothetical is buying a much cheaper battery for some reason. There is no reason to play semantic games with the time value of money, it is clear what the original person was saying.
My point was that as lifetimes get longer, value does not scale linearly. Would you pay 1000x as much for a battery bank that would last until 12019, or would you assume that it would be made obsolete at some point over that period? Maybe we've all moved to fusion, and grid-scale storage is useless. Or perhaps prices decreased by 10x over the next 20 years, and so getting a 20 year battery + a 9980 year battery would only cost 2+998/10=101.8 times as much as the original 10 year instead of 1000 times.
Time value of money is important, and IMO would be enough on its own. But opportunity cost from excluding future improvements matters too - and given the improvements in battery tech over the past decade, one that would certainly be relevant over the next five decades.
