The cost of replacing them is essentially zero though: New ones don't have to be minted because the existing ones are divisible with 8 decimal places, and that

The cost of replacing them is essentially zero though: New ones don't have to be minted because the existing ones are divisible with 8 decimal places, and that can easily be increased. Instead of minting new ones, the value of the existing ones increases, resulting in the decimal point of the value of a Bitcoin slowly shifting. And the amount of currency which humans use typically is so small that storing the numbers to represent them is basically free, no matter if the decimal point shifts a bit. I would blind-guess that 128 bit will probably be enough until the heat death of the universe.