"The batteries are only warranted for 8 years and the...

"The batteries are only warranted for 8 years and the replacement cost is $13,000-$15,000, possibly more."

Is that estimate in 2023 dollars or in 2031 dollars?

And do you think there will be no significant drop in battery prices in the next 8 years?

Here's some US DOE data up to 2021: The Declining Price of EV Battery Packs

There was a significant change in slope at 2013, but there still was a good price drop in the 8 years between 2013-2021.

I expect in the next 8 years that the EV battery market will experience more intense competition than in the last 8 years, with greater pressure on price/performance ratios and on technology improvements. By 2031 the improvements could be so great that it's not worthwhile to replace a worn-out 2023 battery pack, just like it's not worthwhile fixing old Solar PV systems. Or "old" EV battery replacements may become a popular industry for 3rd party companies who can do it cheaper than the majors, or because the majors will not be interested in doing it because it's not profitable enough.