I’m still driving a 26-year old Nissan Micra – though it’s now on its last legs: the Irish climate isn’t kind to steel and we’ve had to have the under-carriage re-welded three times in the past five years. :(
But let's go back to the original point, about being able to UPGRADE (not repair/replace) battery in the car. 20 years old car is worth like $1k-2k, which is fraction of the cost of the new battery.
While it's cool thing to do for hobbyists, it makes 0 economical sense.
What an old car is worth depends on many factors, but age is not the most important one. The average age of passenger cars on the road in the U.S. is 14 years old -- I think 14.5 years now. I don't think we have data on average appraised value of passenger cars on the road, but I would guess it would be in the range of 10-15K.