Hypothesis contrary to fact is a great way to get in trouble. This is the least of the reasons, but not an unimportant one, that Lewis has Aslan refuse to discuss what might have been.
How meaningful can it be to say "If Judas had not betrayed Christ," or "Had Mary not given her assent?" What we know is that they did what they did and it was very important.
Couldn't agree more. However, I was not the one to suppose in print what despair we would have to face had Mary refused God's commission. I think this goes back in a way to the idea of God's predestination versus his foreknowledge. Did God predetermine that Mary would agree so that he had her be born without sin (not my view, BTW)? So, did Mary really have a free will choice? You can thank me later for conundrums that keep one up at night. :o)