[The Might]

Well...everything is life is actually just making choices, same in Tolkien's creation.
Nobody can predict what would have happened had they disobeyed Melkor, it could be that the sacrifice of the faithful would have impressed the others and moved them to making the same choice.
However, we see these people still were capable to change, since not all Men followed Melkor in the end.
Anyway, the story is if I remember well quite old, and so shouldn't be considered as important as Tolkien's later thoughts on such matters.
And also I remember seeing some quote of Tolkien's, from the letters I believem where he way pointing out that if Men weren't mortals, they would just be something very close to Elves, and there wouldn't be much point in creating them as Secondborn as such.