[Zigûr]

A potentially relevant quote from Morgoth's Ring:

"But, if we dare to attempt to enter the mind of the Elder King, assigning motives and finding faults, there are things to remember before we deliver a judgement. Manwë 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. One especial aspect of this is the strange way in which the evils of the Marrer, or his inheritors, are turned into weapons against evil. If we consider the situation after the escape of Morgoth and the reestablishment of his abode in Middle-earth, we shall see that the heroic Noldor were the best possible weapon with which to keep Morgoth at bay, virtually besieged, and at any rate fully occupied, on the northern fringe of Middle-earth, without provoking him to a frenzy of nihilistic destruction. And in the meanwhile, Men, or the best elements in Mankind, shaking off his shadow, came into contact with a people who had actually seen and experienced the Blessed Realm."

So without the Noldor rebellion, Morgoth would not have been "kept at bay". Here we see a more pragmatic characterisation of Manwë than the somewhat naïve image that might otherwise be thought of. The Noldor rebellion produced good.

In this counterfactual, it seems that if the Valar had done nothing then many Elves would be been killed or enslaved, but this quote suggests that Manwë was concerned that if the Valar had intervened directly at that time the provocation against Morgoth might have had an even worse outcome, perhaps the destruction of Arda itself.