But I can’t for the life of me understand how any language designer signed off on one based indexing.
You have to keep track of which language you're using anyway to produce correct syntax...
When building data-structures you're often using small unsigned integers that use the complete value-space, (e.g. u8s and their bitsets in tries, u16s and blocks with that size in sparse vectors).
With 1 based indexing, you can't do that anymore without first having to cast the index into the next larger integer, adding 1, and then doing the access, which is a PITA, adds instructions, and messes with prefetching, while you're already working with difficult, hard to read, and heavily optimised stuff.
It's been a long time i have used any language with one based indexing, but i don't remember it being particularly important in my day to day.