There are other things to consider when doing something like this.
You imply there that a considerable amount of the corpses would remain (it sounds like all of them). Some quick points:
- How's the pathfinding going to handle them (if there truly are many of them)? If the pathfinding uses the pre-baked navmesh of the scene (which they have confirmed it will), then all characters would try to go over/through the bodies constantly. Is the navmesh going to be updated after every dropped body? Or maybe after there are more than two dropped bodies piled up? Updating a navmesh can be costly and if the fights are in the magnitudes of 500v500, there would be considerable amount of updates, often within short intervals. This could eat up the ms budget of the frames.
- How are the collisions going to work? Do the characters go through the bodies or do they step (or even climb) over them? The latter would be pretty hard to program and would probably be a lot of unnecessary work for very little to show for it. This combined with the pathfinding problem is going to be a big headache, however skillful the team tackling the problem.
- Continuing with the previous point, are they going to be static? If, say, a horse would ride fast over a sea of bodies and hit it, would the bodies move? If not, would the horse then fall down like you would expect (if there truly are many bodies, not a couple ones spread around)? Or would it go through the corpses like it was air? Any of the options would ruin either the immersion of the moment OR the fun of the core loop (i.e being constantly tackled by corpses). No winning scenario here unless there would be massive overhauls on the locomotion logic both without a mount and with one.
- The psychology of a player tinkering with the settings: If there is an option for the persistent bodies but together with all the other things that eat up the processing budget it would considerably lower the average fps for most, so majority of the players would feel like they are missing out. Maybe it bums them out, maybe not, but it will affect the psychology of someone who has to either set the setting low, or maybe he will lower some other settings in order to allow for a higher setting on the corpses. Now this can lead to many other complications such as if people feel like the corpses are essential for immersion (provided that a proper way to deal with hundreds of bodies on the ground is even discovered), they will turn the corpse setting up and other settings down, resulting in for example, worse looking clips on youtube or streaming platforms. That could be bad for the public profile of the game, especially so if they use the graphics as a selling point (not sure if they will, but who knows).
This is all to say that having some bodies but cleverly removing them unintrusively will be A) way more efficient in terms of work hours and results and B) visually more immersive if indeed done well and unintrusively. If there are a lot of bodies on the ground and our eye registers a lot of the alive characters just passing through them (which would likely be how it works now?) it
will break immersion. This is not to say that disappearing bodies isn't breaking the immersion, it's just that in the heat of the moment, if done correctly, the player would not actively register that there are bodies disappearing. But other characters constantly clipping through bodies would be an eyesore (as opposed to clipping through a single body in passing, which our eyes likely can deal with provided that around the body there's enough free space that the movement looks natural after passing the body).
I mean it would certainly be cool. In my eyes I can imagine an intense fight with visuals akin to the GoT battle of the bastards. Is it likely to be successfully implemented in a game that has a million other things going on at the same time in a dynamic, open-ended battlefield? Not right now, no.
Edit: The reason why they might not want to expose that to players/modders is maybe they have a very finely tuned clever solution to the above problems that also partly exist with limited amount of corpses. Maybe they have realized through testing that they can keep a certain specific amount of bodies before either the disappearing starts to break the immersion or the other problems would start to break it.