How would it all be?

I think that had Morgoth been successful in corrupting men of the three houses of the Edain that the first age would have turned out completely different. First, the men and Elves were utterly defeated by Morgoth anyway, and only through the intervention by the hosts of the Valar were they saved. Second, the intervention by the host of the Valar was a direct result of the voyage of Earendil the Blessed. Had men been corrupted by Morgoth, Earendil would never have been born, as his Father Tuor wouldn't have met his mother Idril in Gondolin. So based on that, had Morgoth been successful with his plan to corrupt men, things would have turned out far differently.