I was planning on an answer almost as Eric's which will now carry no weight so I'd focus on possible consequences if man had played Jesus' role.
First of all I do not think we could compare man's 'perfection' (Mary or anybody else) to that of God.
"There was a man in the land of Uz, whose name was Job; and that man was perfect and upright, and one that feared God, and eschewed evil." (Job 1:1).
Does this mean Job was sinless throughout his life? This is what our bible says about him, how much more a doctrine created by a 'brand' of christianity, a belief that man thinks reflects what is in the bible.
As righteous as man could be, he could not understand FULLY the plans of God. We know what God's main purpose is but we do not understand all his ways, methods and even time of accomplishments.
Let's get back to Job for instance. He faced harsh times but...
"In all this Job sinned not, nor charged God foolishly." (Job 1:22).
No doubt about his faithfulness to God but he began to degrade his significance under the eyes of God. Though he did not charge God foolishly, there were wild and deeper complaints that pushed God to remind him of who he (God) is.
"Then the LORD answered Job out of the whirlwind, and said, Who is this that darkeneth counsel by words without knowledge? Gird up now thy loins like a man; for I will demand of thee, and answer thou me." (Job 38:1-3).
After a long, strong reply from God, Job confesses:
"Who is he that hideth counsel without knowledge? therefore have I uttered that I understood not; things too wonderful for me, which I knew not." (Job 42:3).
When we know that His ways are not our ways, it tells us that our worldly desires (by default) cannot allow God's will to totally be done on earth, even when it is our prayer. Even Moses, whom I consider as one of the most active and closest prophets of God, sometimes lacked boldness, and failed to carry out 'simple orders', not to mention Jonah.
Can we compare these to Christ (God)? When the time for all the physical torture to begin, Jesus felt the heaviness,
"And being in an agony he prayed more earnestly: and his sweat was as it were great drops of blood falling down to the ground." (Luke 22:44).
yet He asked His will to be done.
"He went away again the second time, and prayed, saying, O my Father, if this cup may not pass away from me, except I drink it, thy will be done. " (Matthew 26:42).
His death was not a one time killing by bomb blast which others (martyrs) will claim to die in for their course. It isn't a gunshot that will limit the period of pain before death. This One is spat upon, whipped, crowned with thorns, among others, finally hanged by nails on a cross and left to die.
For humans, the thought and expectations alone, knowing that all these will happen are torture enough to make us NEED an escape from such fate. We may be tempted to argue with God: Why at all must I suffer all these before I die, why not death by the sword in the garden after Peter reacts? And a whole lot, simply because even when we believe in Him, we cannot comprehend fully and allow God's will to be done without questions. Only He could.