It never made sense to me Sanji ripped off Geppō but not Rankyaku.
Oda was one some hater shit with decision.
Not only would ranged attacks add new dimensions to his game, specifically being able to cut things with a Rankyaku-like technique would allow him to expand his cooking-themed attacks.
Chopping, dicing, mincing, Julliene, Baton, Mandoline, Brunoise, Chiffonade - the list of variety can go on and on. With or without flames.