Except, it's not. To fight out of sheer defiance is to accept that there's no chance of our victory, to cede that Morgoth will eventually conquer all and that all there's is in our fate is to delay the inevitable as much as we can, because that is the only way we can live. There is a certain strength in accepting such doom and deciding to fight on regardless, it's true, but it is not stronger than fighting with knowledge that however mighty Morgoth may seem, there are powers much stronger and things that he can never hope to subdue, and that those things and powers are on our side. A strength from defiance seems brittle to me, because it's a power that burns spirit with every tribulation, and even the mightiest will eventually spend it all, much like Turin Turambar did. And if even the mightiest heroes run out of spirit to continue, I do not see how the entire civilization can not. A man cannot live without hope.
And it does seem very Feonorian, for what was their oath and then insistence on keeping it and continuing their pursuit even despite the Doom of Mandos, if not an attempt to defy the will of both Morgoth and Valar? And while they were unquestionably mighty in their defiance, it made them lesser, in the end.