Are there any explanations for the 五段動詞-specific イ音便 of 連用形+た/て that aren't based/dependent on the 子音語幹/母音語幹 explanation for the differences in 五段動詞/上一段動詞 conjugations?
For example, take 開く and 飽きる, setting aside the fact that they have completely different meanings and focusing only on their pronunciations. In 口語, the 過去形 of あく is あいた, due to the イ音便 of "あき+た". However, for the 上一段動詞 "あきる", this イ音便 doesn't occur, and so the past tense is "あきた".
I've heard explanations regarding 子音語幹 and 母音語幹. However, they don't seem able to explain why the イ音便, which occurs based on how difficult a word is to pronounce, would not occur indescriminately. Is anyone aware of any other explanations?
In other words, if the past tense of 開くwas reanalyzed as 開いた, why was the past tense of 飽きる never reanalyzed as 飽いた？