"0123456789"

  ch[0] = 0;
  ch[1] = 1;
  ch[2] = 2;
  :
  ch[9] = 9;

  ch[1] = 0;
  ch[2] = 1;
  ch[3] = 2;

....

Please re-read his question: "how many chars we have?" "how many elements we have?"

  ch[1] = 0;
  ch[2] = 1;
  ch[3] = 2;
  ...
  ch[10] = 9;

This is about having the Nth element of the array with the index N rather than N-1.

That's not code, that's English.

When we start indexing from 0, the only time we need to do a +/- 1 is when we need to index the last element. All other times (iterating, mapping) we don't need to.

Also, starting indices at 1 would change the idiomatic iteration to:

   for (int i = 1; i <= SIZE; ++i)

I find that the <= and >= operators add more to my cognitive load than the strict relation operators < and > because there's an or in there. I don't find the alternative to that idiom any better:

  for (int i = 1; i < SIZE + 1; ++i)

Your original argument was that we should make it easy for humans to understand, not computers. I think that starting indices from 0 is easier for humans to understand because it simplifies the code we must write and read.

No way I would trust a language design with your semantics for this syntactic construct: for(i=1;i==n;i++)