I think had everything gone in their favour (eg. Thingol aiding them, and some of the men remaining faithful rather betraying them), they may have defeated Morgoth. Like it said, Morgoth was forced to use his last reserves. At one point they were banging on his gates and he withdrew in fear.
It was not to be, however. They were not fully united and some of the men did betray them. Mandos would have seen this, and hence knew their doom. I don't think he was saying to them, "You have no chance," but rather, "I have seen your future... things will go wrong and you will lose." Under different circumstances I think they stood a chance. Okay Morgoth was a fallen Valar, but Fingolfin gave him a good fight in a single combat. Imagine a legion of elven heroes trapping him in one of his chambers... bloody, but they'd probably manage to overcome him. By creating Morgoth's Ring he had effectively bound himself to flesh and the earth, so he was assailable.