Yes, if the Nirnaeth had been won then Morgoth would have been defeated. It was the most decisive battle of the war. Finwe is quite right; Morgoth could have been defeated were it not for the disunity of his foes and the treachery of the rogue Easterlings.
Remember, Morgoth was several times stronger when the host of Eonwe attacked him than he was in the Nirnaeth. He had swelled his forces, bred dragons and made his control of Beleriand almost universal by the time of the Great Battle. Eonwe had a greater foe to deal with than the Elves did in the Nirnaeth.
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