I feel like the "refillable" Copic pens are a bit of a marketing ploy. They seem like they would be better for the environment, but you don't refill the ink chamber, you replace it. The cylinder of plastic you throw away is the size of a pen body, so it really doesn't seem much more environmentally friendly. The only time it seems like you're throwing less away, is when you replace just the nib. I suppose if someone went through a lot more nibs than ink it might be a better option for the environment... but that hypothetical person seems unlikely.
Perhaps the plastic on the ink cartridge is a lot thinner than a plastic marker body. I haven't held one, so I can't speak to that.