The declaration that the stories are allegorical is a categorical statement. Are categorical statements — statements not allowing for an alternative — not requiring of some evidence for their assertion?
I understand what it is you are saying, but personally I do not see how a one-time sacrifice by Jesus Christ, a sacrifice that is an allegory, has any meaning for sin or righteousness. If Jesus is an allegory we are still in our sins, and the name of Jesus Christ does not have any power to save.
How exactly do you respond to the statement,
“the Word became flesh and dwelt among men, full of grace and truth?”
If the statement above is not true in time and space, personally I will cease to be a Christian the moment I am convinced of its falsity.