Thanks for taking the time. It's an interesting argument that many people live past the point that they enjoy life, but I doubt you are really suggesting we would be right to kill those people without their direct consent. Euthanasia, where it is at all accepted as moral, typically requires consent. If you think you can get the consent of an animal to kill it, well... let's cross that bridge when we come to it! Regardless, 'death' in this instance is still not necessarily a pleasure -- it may just be the option with the least suffering, and even if there are exceptions to death as a form of suffering, it doesn't seem far off base. As living beings, we value life.
Next, I agree that "natural" could mean a lot of things. In this case, understanding that we're discussing moral involvement, I'm speaking about whether or not you have moral involvement in causing the death.
Paying for a burger from McDonald's is indirectly asking for the death of the cow. They wouldn't kill it if no one bought burgers, but we all know that people do buy burgers, so the blame is diffuse. We all share. But you, in eating it, still profit by the being's death, and it was ultimately a deed done for your benefit when you chose to pay them. Of course it is more direct if you pay a butcher to slaughter a cow, and far more direct if you wield the knife. Hit a deer with your car by mistake? Maybe that's like manslaughter levels of culpability.
As a sidebar, is a cow killed by a wolf a natural death? It happens 'in nature', and the wolf couldn't live without killing other things, so it is natural for a wolf to kill a cow. But... is the death a natural death? I do not feel it is. I cannot blame the wolf (as it does not know better and does not possess other actual options, like we do), but I do not think the cow exists for the wolf. To paraphrase Kant I think, each being is an end unto itself. The cow's most natural death is one of old age, i.e. what we would legally call 'natural causes'. Suppose that health care could have extended its life, well sure, but there's a limit of practicality.
Last, it would be possible to justify literally any act from the position of "maybe this is just how the world works and we have to accept it" -- so, again, sure. But slavery was 'just how the world worked' for a long time, and I feel I am better off, and I can think of a hell of a lot of other humans who are better off, for us eventually having not just accepted it. There are similarities.
Veganism is really the only choice that's morally consistent, intellectually honest, and doesn't rely on special permission. It is ultimately not really arguable that it is a terrible thing for us to do to breed them, to keep them, to manipulate them, to harvest their products, and to kill them, especially in the ways we do. One must rely on ideas like 'might is right', 'we are inherently special for reasons that must be taken on faith', or 'nothing really matters anyway, everything is relative', which are exactly the arguments that have been used to justify all past atrocities of our doing.