You are giving far too much credit to internal conflict. Trapped between an empire of orcs on one side and those super racist elves on the other side (they certainly do consider humans a lower species, and how could you blame them given the humans' shocking lack of eternal youth), how much internal conflict can you expect? Even the most incompetent dictator wouldn't have a problem getting people to stand together.

Most older Elves regard being able to leave the world as a kindness, given how twisted it has become. The racist ones are either bitter or don't have much life experience.

Elves are not only long-lived, but also bound to Arda. When they die, they rest some time in Mandos' halls and then reincarnate. No matter how twisted and shitty the world becomes, they are stuck with it. No matter the traumas they suffer and the wrongs commited by them, they cannot escape it [the consequences]. Because their victims will be around forever too, and they can't forget either.

I'm not sure I get you. We were talking about possible internal conflict within the Orcs. Not sure Sauron gets their automatic allegiance, and if he does not, that's all the reason we need. As for humans, I don't have a good explanation there at all.

I was referring to the second paragraph. Orcs usually don't pay taxes to Gondor.

That paragraph referred to humans. For Fantasy races we can always come up with an explanation/excuse, but there should have been more humans along the route.

Politics is handwaving for the structural reasons for a conflict. No need for fighting each other if there is an easier gain to be found outside. At least not fights that would be significant enough to suppress the population growth.