We chat about videogames and videogame accessories.

Like I said, that comes at the opportunity cost of creating a whole new story (or world)!

I could turn this around and say that creating a whole new story/world comes at the opportunity cost of further developing your existing world.

Guess what? Neither of us is wrong! It turns out both options have costs, which is why sometimes creators will pick one and sometimes they'll pick the other. You have not explained why you seem to think creating a new world is inherently better. I don't think either option is; a given creator considering a project will sometimes decide that further development of the world they've got is a better option and sometimes decide that making something completely new is.

Also, there's no reason why you can't do both over the course of your creative career. The opportunity cost, whichever direction you point it in, only applies to "the project I am currently working on".