In what way does treating the Bible non-historically mean a low opinion of the Bible?
Firstly, note that Genesis almost doesn't state that Genesis is historical. Luke says he puts together an ordered record in his Gospel and in Acts and we have no bones to pick with that. Samuel, Kings and Chronicles apparently make references to outside historical sources the way one textbook might cite another, and we don't refer to them as non-historical or diss the miracles there as impossible. But does Moses say Genesis is exact history? Maybe not.
Secondly, you are interpreting this through a Western mindset of objective historical truth. To the Enlightenment - Rationalist view, nothing is true unless it is a fact i.e. it actually occured in such a way at a verifiable date and time. However, some things that are not facts can be and are true. For example, the Good Samaritan story might never have happened. Does that mean I think Jesus was a liar? There are many non-historical / non-literal forms of truth, like:
-parables
-poetry
-allegory and fable
-figure of speech
Saying that the Creation story is merely an "allegory" may disturb you, but it surely wouldn't disturb me (even though I'm not going that far yet). It is not the insult you imagine it is. From the Chinese culture there is a wealth of tales and fables. For example, Romance of Three Kingdoms which purports to be a historical record, what with dates and battle records, but also having funny things happening like summoning the wind and a pious official surviving without his heart for some time. Other stories go even farther out ... the point I'm making is, nobody takes these tales seriously as history, but these tales are still at the heart of Chinese culture.
In the same way, even if Genesis is not truly history it is a vessel of divine truth. And that insults it how? ...