I don't know if I agree that it "greatly" affects immersion. If I shoot a dryas elk with an arrow and it falls down, I don't need to see blood to know it is dead. It's lying on the ground, an arrow is sticking out of it, and I have the option to harvest its meat, organs, horns and hide. I'm not particularly averse to a splash of red around a wound, but I don't think it's vital to my gameplay experience.
There is also the matter of technical hurdles. There are quick and easy ways to give the impression of blood during an attack, but if you're looking for persistent gore, that's a whole other layer of work they'd need to do.
You mention there are some people with a genuine aversion to blood. In that way it also comes down to a matter of tone. What is the tone of the story. How should the player feel about their accomplishments? What do they want you to be immersed in? What will the ESRB label this game and how will that narrow the playerbase? Being able to toggle it on and off is another technical hurdle, but if it is always on or off they aren't "forcing a preference" they are dictating the tone of the game--something they do with all of their world-building choices. It's impossible to avoid.
Caspian has said he doesn't want the tone of the game to rest on gore and sex, but rather the stories and adventures we build together. If you're looking at visceral brutality, I don't think that's what this game is about. I also think that there are numerous opportunities to increase immersion already, and that once we're in the game nobody will truly miss immersive bleeding.