I agree with your reasoning but ‘dure’ is not a free morpheme in english, isn’t it ? It’s supposed to be a bound morpheme too. Yet dictionaries say that ‘during’ comes from an obsolete verb ‘dure’ which actually was a loan word from latin via old french ‘durer’ (to last). By the way in contemporary french ‘durant’ is the present participle of ‘durer’ as well as a preposition. In some ways it calls into question the synchronic/diachronic distinction. It’s well-known that lexicon is the area where that distinction is the most problematical.
I remember two old papers written by R.Jackendoff and M.Aronoff. Aronoff in particular wondered if forms like -sist (assist, insist, persist, consist...), -fer (prefer, confer, infer, transfer...), -mit (permit, commit, admit...), per- (persist, permit, perceive...), con- (confer, consist, consume, conceive...) are morphemes in english. All are latin verbs. He concluded that they are meaningless morphemes (which he ended up calling ‘morphomes’ few years later if I’m not mistaken, I don’t remember well). I think it’s indirectly related to your issue since I think we can find one arguing that these forms have or had a constant meaning.