Fluid and natural are pretty different concepts, perhaps "intuitive" better maps to what you mean? Humans aren't that fluid, certainly wouldn't be when it comes to vaulting, jumping etc.

Playing a game with realistic FPS movement like milsims is a totally different experience.

When it comes to fun and intuitive movement, I would say realism should go straight out the door. I want to feel like a cheetah chasing a goat across a cliffs edge in games. Personal preference but I feel like objectively more fun.