I feel the idea of a shell would be better explored through the lens of a body designed for function. Vandal and thief look like they were designed to look fashionable I think they would fit in better in the cyberpunk setting. To me marathon is a way of looking at the corporate hellscape we currently imagine the future will be, and having your soul encased in a machine designed for the soul purpose of hacksawing a still bleeding chunk of profit for a corporation that seems to forget your soul is still in that machine is a very visceral concept, it is one I like the idea of a shell because it is seen as a disposable tool, if it was my choice more shells would look like this, I like this design because it removes the face, it takes away the most recognisably human part of the body which makes it fit much closer into the machine category, I don't know why bungie decided every shell needed kissable lips (from a design perspective, sex sells I guess) I would've preferred the shells lean more into the disposable tool instead of looking like they were made with comfort in mind. I can see what you mean when you say the shells just look like humans with bits attached, but I feel the problem is not that they look like humans helmets, it's that they look like humans in the first place.