I agree with you,
Gollum. Even if he had not entered Middle Earth itself some other form of evil would arise, such as corrupted King, evil Mia ect. The interesting thing about Morgoth was that he did not just seek to enslaved Middle Earth itself, but he thought that he could destroy Valinor. Now when we see people like Sauron, who was not a god like Melkor, I don't think that he had any intention of traveling across the seas to lay siege to Valinor. This might have something to do with that Melkor was not once a mortal, like Sauron was, so he could not just be content with all of Arda.
The gods role in evil
The gods saw Melkor as a threat so they waged war on him, basically he could for their own protection. Whereas they let Sauron be, letting the races of Middle Earth take care of themselves (I bet Ulmo had something to say about that
). That's another what if question, would the gods have eventually destroyed Sauron like they destroyed Melkor (I know I'm off the subject), or would they have let Middle Earth fall into darkness?
However if Melkor had not come to Middle Earth I very much doubt that the Valar would have the need of embassies to Middle Earth, such as Curumo and Olorin. Because, the way I see it, Melkor breeded mystical evil that would need to be countered in future generations, therefore they sent the five wizards to help the peoples of Middle Earth.
Good question,
Gollum the Great.
