When is level streaming needed to supplement occlusion culling?

Ok so lets assume I have a decent sized world that compromises of eight or nine large outdoor terrain tiles blended together at different heights.
I want to know if this system is viable for a shipped product without the use of any streaming or world comp if each terrain occludes the other terrains assets from view completely.
Would I theoretically be able to get away with this? I only want to stream things that need to be unloaded. So i figured unloading NPC’s in terrains you are not inside via volume propagation would be essential however i couldn’t think of much else that would NEED to be streamed if the occlusion culling culled absolutely everything from being rendered.

*Perhaps someone here has a better recommendation for best practises when rendering large worlds for a 4.22 single player game that wants no loading screens or load times between each terrain. *