When was this rule to prolong short vowels added to Japanese?
And is there any formally accepted reason for using う and い instead of お and え? (which would seem like a more natural option, in my opinion)
There are examples where お is used after a syllable ending in ~O, are these phonetically equivalent? とお(遠) and とう(塔)?
Edit: My question, specifically is why is う used to make syllables ending in O long, when a more intuitive and natural option would have been using お (And this premise is totally subjective and my opinion, if someone disagrees, please explain)