The short answer: Tetsuya Nomura.

He was heavily involved in the designs for Final Fantasy VII, VIII, X and XIII, some of the arguably more "space-age" titles in the franchise.

Quote Originally Posted by Sapphic View Post
The worlds have been bent more towards futuristic with fantasy swords and sorcery elements.
I think you'll find that a number of fans of the franchise would disagree with your impression, especially the older ones who have played Final Fantasy games since the beginning. For many of them, the move towards futuristic designs was a departure from the norm, and that's why FFIX was so popular: they regarded it as a return to the franchise's "sword-and-sorcery" roots.

In any case, you'll find that almost all Final Fantasy titles have some sort of "futuristic" elements in them. I, for one, am not overly fond of space-age elements in my fantasy role-playing games unless they are part of its integral design. My favourite title in the franchise, Final Fantasy Tactics, does not feature any faux future designs, and that's likely part of why I love it so much. Contemporary fans liken it to the Game of Thrones, which is an apt comparison. Back when I first played it, I simply loved dark and gritty tone. It was a game filled with ethos and pathos. The other Final Fantasy titles I enjoy are very much in the same vein.