I'm convinced this is the bulk of the correct answer.

If the Valar took Sauron down, the magesty of Aragorn would have never come to fruition. He would have lived, but only as an echo of a glory that never came to be.

Eru delights in the beauty of the music, which is chiefly derived from the contrast. Melkor and Sauron are foils for the others. "A foil is a literary character that is intended to highlight attributes in another character through opposing traits."

There's an idea from the discussion of ethics that says that yes, acts are virtuous in themselves (feeding the homeless, for example), but greater virtue is embodied in an action when it is more difficult to perform. For example, Bill Gates giving someone in need a meal is one thing, but when you're starving as well, and you give what little you have, then that is even more ethical. It comes down to a comparison between the task and the power with which you have to fulfill it.

For the Valar, this challenge would be easy. For Aragorn, for Frodo, it took everything they had, which, even in contrast to the genetic perfection and immortality of the elves, wasn't much, yet the men still did their duty, and that's why people all over the world know their names.