I shudder to think of the entity count if everything was infinitely persistent (bodies, trash, etc). I haven't looked into how private servers will be hosted, but the server load will increase exponentially as more items are placed in the world. If I were to host my own server, I'd definitely want items to despawn from the world on a variable timer that increases with the value of the item dropped. Bodies would ideally stay in the game long enough to pull items from it or use it however the player wants. If the body lies untouched for, 30 minutes I guess, the body would despawn.
The developers could choose to go a completely different route, but most games try to reduce the entity count as much as they can without affecting immersion.