The last line of the prophecy never stated who would win the fight. Just that it would essentially boil down to Alduin and the Dragonborn.
Anyway, there is, ultamately, free will. All prophecies are written and seen in the Elder Scrolls. Each Scroll, however, can show a different viewer a different prophecy or outcome. No two people will see the same events in a prophecy, or even the same prophecy when reading a Scroll. Anyone with the knowledge of what seems to be written in an Elder Scroll can try and force the Prophecy, as seen, to be fufilled. Once so fufilled, the Elder Scroll becomes "locked," no matter who looks at it, the prophecy written on the scroll will reflect what is written in history. No amount of magic can change it afterwards.
It should be mentioned that the Elder Scrolls are reality-warping artifacts. The one used in the TG questline retroactively removed the Curse of Nocturnal's Cowl, resulting in the Theive's Guild's fortunes reversing immedately. And I mean the curse was in effect up until the end of the Questline... and then, to anyone not in the know, there was no curse.
We can not discuss real world religons on these boards, do not ask for that, nor should anyone provide their religious affiliation here. You can talk about the religions of the game, that is good fodder for a discussion.
As for not defeating Alduin. No MQ, or any quest for that matter, is required by the game. A large number of people ignore the MQ either in part or completely, because they can. Yes, you can lose the battle with Alduin, but the game will just reload to your last save in that case. So, if someone were to travel to the final area for the MQ, then save, then they would have to resolve the MQ in order to play other parts of the game. Unless they had a save to reload from before they went there.
Err, I think you misread the OP...