Corruption and Evil... leading to power loss?

[Iarhen]

We can see, both in the Silmarillion with Melkor and in the LOTR with Sauron, that both the Dark Lords were terrible in might, but their power diminished with the passing of time.

Melkor first. From being the most powerful of the Valar, his power diminished to a point where he was hurt by Fingolfin, he was enchanted by Luthien and was utterly defeated... Why? I think I remember Tolkien saying that because of his corruption (along the extertions, the will he had to place forth among his servants, etc) his power diminished.

That's the same that happened with Sauron. From being the most powerful soldier of Morgoth, he is destroyed by the mere melting of his Ring. His corruption obviously affected his maiar-powers, and utterly diminished to a point where an elf was able to read his mind (Galadriel). Besides, if his Boss Morgoth suffered that loss, being more powerful than Sauron, I pressume that Sauron suffered the same doom.

What do you think?