Throughout this season of Supernatural, we've heard a lot about destiny. Gabriel tried to get the boys to recognize that there's only one inevitable outcome in "Changing Channels," and Michael once again tried to hammer the point home in "The Song Remains the Same."
Dean can't change his destiny. It's gone back thousands of years, and every little seemingly random event has led him to this point. Every time he tries to stop it and run away, he just gets closer to the end result. Dean will say "Yes" to Michael, Sam will say "Yes" to Lucifer and the two angelic brothers will have their Apocalyptic showdown.
Is there any way out of this? The short answer is: no. Supernatural is a show built on destiny. It's a show where the creator announced early on that he had a five-year plan, so literally everything has been leading up to the finale of season 5.
Sam tried to stop Dean from going to Hell, but he did anyway. Dean tried to stop his torment, but instead he opened the First Seal. Sam tried to do the right thing and kill Lilith, but he opened the Final Seal. The entire series is about Sam and Dean's quest to stop the inevitable.
To understand the futility, just look at every season finale.
-The Yellow-Eyed Demon escapes and the Winchesters are in a car crash
-Sam dies, Dean sells his soul and the Devil's Gate is opened
-Dean goes to Hell
Supernatural doesn't have the best track record of preventing major disasters. Perhaps this was Eric Kripke's version of the rope-a-dope. Maybe all this pounding was a way of making it all the more triumphant when the boys actually succeed.
But I'm still on the side of Michael. Destiny seems pretty convincing and maybe Michael was right and free will is an illusion. Did Dean voluntarily sell his soul, or was it part of a larger plan, setting up all the pieces in his life in the perfect formation so that Dean only thought he did it of his own free will?
Dean may think he can say "No" to Michael, but the plans are in motion. He will say "Yes," and when he does, it won't be his choice, it will be the way it was always meant to be. Free will really is an illusion on Supernatural.