Never said performance wasn't shit lol. It definitely is.

It's just that you pretended to know what you're talking about. Occlusion culling is done only for rendering the scene. That is, the object remains in the game's memory, but it will not be drawn by rendering calls if there are other objects visually occluding it entirely. That way said object can still be used for things like path-finding, AI, and physics interactions. If you expected an object to be unloaded from memory than reloaded every time it was visually occluded and un-occluded then you would have the choppiest game imaginable, because hard drives, even SSDs, are still the slowest component in a computer.

LoD can be done in any game and works just fine for any game. The key is that the transitions happen smoothly and far enough away that the player doesn't notice. Most top down games have cameras that are too close to the objects being rendered to use it effectively. In effect, I agree that LoD is easier to use for first/third person games.

That being said, your typical third/first person camera is still going to be rendering more geometry even with LoD due to how far the camera can see. It does vary scene by scene, but generally holds true. You've looked out over the open wilderness in a Bethesda game yes? Even a street as you've said, will see more objects looking down it than from above it, unless of course you're including the rooftops of multiple buildings around it and it's a short street. You need niche scenes that would have more geometry for a top down camera than one looking out over a vista. I'm using XCOM as a reference point here. Obviously you'd get a huge amount of geometry if you zoomed out over an entire map, but then LoD can start to apply ...

But what does this have to do with XCOM 2 anyways? If anything its shit performance is probably due to unoptimized shadows and lighting.