the lord of the rings - What would've happened had Melkor won?

I'm interested to know if there were any notes on what would've happened has Melkor successfully overthrown the other Valar and assumed complete control of Arda.

Obvious answers like "He'd kill everything and make more orcs" are not what I'm looking for here, more looking into the possibility that it was impossible to win and therefore his efforts were completely futile. Yet surely, a spirit as powerful and wise as Melkor, having listened to the music, would be able to distinguish the difference between something worth doing and something that's not.

I ask this because surely, Eru being the giver of life and all powerful would not have created something that would have the ability to overthrow even himself? But yet Melkor persisted in trying to assume at least equal standing after his discord in the music and continued to do so after Arda was created.

Few things to answer here. Was it futile? Were there any Tolkien notes on it? If not, anyone else who was close to his works like his family.

Answer

Surprisingly, Tolkien did actually write a bit about this. In an essay titled "Notes on motives", where he compares and contrasts Sauron and Morgoth at the height of their influence, there are hints at what Morgoth would have done if left to his own devices. The key points:

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  It's not really possible for Morgoth to "win", because his goal isn't achievable. Morgoth's "goal", insofar as he has one aside from random villainy, is described as:

  
  

  
  

  Morgoth had no 'plan': unless destruction and reduction to nil of a world in which he had only a share can be called a 'plan'.

  
  

  ^{_{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion"}}

  
  

  
  
  

  But this isn't something he can actually do; as Tolkien writes, Morgoth can only destroy the form of matter, not the matter itself:

  
  

  
  

  Melkor could not, of course, 'annihilate' anything of matter, he could only ruin or destroy or corrupt the forms given to matter by other minds in their sub- creative activities.

  
  

  ^{_{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion"}}

  
  

  
  


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  The closest we can come to a "win" condition for Morgoth is ending any opposition to his destruction; if he were to crush the resistance of Elves and Men, and if the Valar retreated and left him alone, then Morgoth's reaction would, basically, have been to eventually destroy everything; but even that wouldn't really be "winning":

  
  

  
  

  Morgoth would no doubt, if he had been victorious, have ultimately destroyed even his own 'creatures', such as the Orcs, when they had served his sole purpose in using them: the destruction of Elves and Men. [...] even left alone he could only have gone raging on till all was levelled again into a formless chaos. And yet even so he would have been defeated, because it would still have 'existed', independent of his own mind, and a world in potential.

  
  
  

  ^{_{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion"}}