that probably would not have made much of a difference. People would have assumed that this would never happen and would have made practical implementation assuming a fixed 32 bit space. By the time it became a practical problem, we would have had a creep of devices that does not follow the norm, and managing that would be a nightmare.
Just see the sorry state of utf-8. We still have so many code bases that are not utf-8 compliant despite we have seen the need for it over 15 years ago.