You're using a file format designed three decades ago to make arguments about what we should be doing today.
XLS as a format sucks because:
a) It was undocumented for most of its life
b) Undocumented even inside Microsoft because it consisted partly of memory dumps from the app
c) Was heavily optimised for fast loading and saving on very slow machines
100% of uses of JSON, translated to binary, would not suffer those issues. They'd have documented schemas, at least internally, they wouldn't be created by memcpy to disk, and they wouldn't be stuffed with app-specific loading optimisations.
XLS as a format sucks because:
a) It was undocumented for most of its life
b) Undocumented even inside Microsoft because it consisted partly of memory dumps from the app
c) Was heavily optimised for fast loading and saving on very slow machines
100% of uses of JSON, translated to binary, would not suffer those issues. They'd have documented schemas, at least internally, they wouldn't be created by memcpy to disk, and they wouldn't be stuffed with app-specific loading optimisations.