The Great Darkness

[Kuruharan]

But it was no orc-chieftain or brigand that led the assault upon Gondor. The darkness was breaking too soon, before the date his Master had set for it
-The Battle of the Pelennor Fields

I think that indicates Sauron was using the darkness as a tool for the war. I don’t think he intended to shroud everything in Middle earth with perpetual physical darkness.

Remember that much of the landscape of Mordor that we see was determined by being next to an active volcano. That will have a detrimental impact on most anything.