Lord of Gravity

[Formendacil]

Why don't the Valar do anything? Are Manwë and Varda powerless?

I think the answer is, for the most part: yes.

Think about it; we are told that the Valar did not make war on Morgoth after his escape from Valinor (I believe this may also be from Morgoth's Ring), quite largely because to do so would rend the earth in much the same fashion as the tumults of the world when they imprisoned him the first time, and they feared for the survival of Men, who are not as hardy as the Elves.

Additionally, though, if we continue to compare the Earth to the Ring, then remember how Sauron's connection to the Ring was finally broken: by destruction--utter destruction in Mt. Doom.

If the Valar destroy "Morgoth's Ring", we are left with... nothing. Arda will be no more--or no more inhabitable by Men, anyway (and probably not by anything else, either).

Of course, the day will come when the Valar must take the battle to Morgoth. Tolkien's writings are lightly scattered with references to "Dagor Dagorath", the battle at the end of times, when Melkor will at last be slain, but also when the Earth shall end. Shall "Morgoth's Ring" be destroyed--the final end of Arda Marred?

[Raynor]

Originally Posted by Formendacil

Shall "Morgoth's Ring" be destroyed--the final end of Arda Marred?

There are two versions that I know of: healing - or recreation of Arda:

Originally Posted by Note iii to Of the severance, Later Quenta Silmarillion, HoME X

Since the Elves (and Men) were made for Arda, the satisfaction of their nature will require Arda (without the malice of the Marrer): therefore before the Ending the Marring will be wholly undone or healed (or absorbed into good, beauty, and joy).
...
But others use another analogy, saying that there will indeed be a New Arda, rebuilt from the beginning without Malice, and that the Elves will take part in this from the beginning.

Originally Posted by Athrabeth Finrod ah Andreth, HoME X

- [Those of the Old Hope] say, answered Andreth: they say that the One will himself enter into Arda, and heal Men and all the Marring from the beginning to the end.

Originally Posted by Lalwendë

"but don't worry, because whatever nasty stuff Morgoth has put into Arda, ultimately only serves to glorify me a bit more".

The more consistent answer is, rather, that all evil works for good, something you touched upon in your post. Tolkien had this idea about his created world

Originally Posted by Notes on motives in the Silmarillion, Myths Transformed HoME X

Manwe was the spirit of greatest wisdom and prudence in Arda. He is represented as having had the greatest knowledge of the Music, as a whole, possessed by any one finite mind; and he alone of all persons or minds in that time is represented as having the power of direct recourse to and communication with Eru. He must have grasped with great clarity what even we may perceive dimly: that it was the essential mode of the process of 'history' in Arda that evil should constantly arise, and that out of it new good should constantly come.

or real life:

Originally Posted by Letter #64

All we do know, and that to a large extent by direct experience, is that evil labours with vast power and perpetual success – in vain: preparing always only the soil for unexpected good to sprout in. So it is in general, and so it is in our own lives. ....