The logic is basic
- Luffy and Zoro are Natural Born Fighters, got a teacher and left TS at Commander level
- Sanji is Natural Born Loser, in flashback he was weakest among Judge's sons, got a teacher (Ivankov) and left TS as jobber level
Sanji can only be relevant with Judge's bloodline
He could be relevant pre TS, but the world is now bigger with New World and Sanji can't compete anymore without his family power
Don't give excuse about "no teacher", he got 1 too
He is just a loser
Jinbe and Lucci spent timeskip without teacher too but they are still relevant because they aren't losers
- Luffy and Zoro are Natural Born Fighters, got a teacher and left TS at Commander level
- Sanji is Natural Born Loser, in flashback he was weakest among Judge's sons, got a teacher (Ivankov) and left TS as jobber level
Sanji can only be relevant with Judge's bloodline
He could be relevant pre TS, but the world is now bigger with New World and Sanji can't compete anymore without his family power
Don't give excuse about "no teacher", he got 1 too
He is just a loser
Jinbe and Lucci spent timeskip without teacher too but they are still relevant because they aren't losers
Oda didn't show anything related to Iva and him teaching Sanji, unlike the thing with Ray & Mihawk. The flashbacks & events in that kingdom only involve the Okamas & recipe master thing.
Also, all mentorship in One Piece are always treated as positive by Oda. That didn't happen in Sanji-Iva case. Especially since one big moment in Sanji's intro is about expressing gratitude about being taught things.
You can label Sanji as loser and weak because there are indeed panels drawn by Oda showing that (Doffy/Vergo/King scene, Germa flashback). But it is also a fact that there is no panel showing Iva being Sanji's teacher.
Having opinion on Oda's actions for the char/plot is fine, but revising it is not.