I see this sort of feature as pretty low priority. Larian still has much to do with building out races and classes and balancing what is already there. I can only see emersion stuff like this being prioritized after the game is in a more complete state. Bodies disappearing with items left behind is my best hope for something being implemented soon.
It's probably more of a performance issue than an immersion issue in the end. Every instance of a body and blood is something the game has to remember. Now how big a factor that is depends a lot on the engine, and I don't know enough about that to say it's going to have a big impact, but we can say for sure that it is going to add up.
Isn't every instance of a body and blood already something that the game has to remember? It has to actively display them every time you get close. I would think that having less of those to remember by periodically removing them from the game would actually be helpful from a performance standpoint.
Edit: Re-reading what you wrote, we might be saying the same thing. I'm not sure if you're talking about keeping track of bodies is a performance issue or keeping track of decay is a performance issue. I wrote assuming the latter.
