Saturn could have looked strong against somebody else. Why does it always have to be Sanji?
1. Luffy : when Luffy became a hypetool no one care about it because he will be guaranteed to "get over it".
Thus, using Luffy as hypetool won't actually increase the tension of the situation.
2. Zoro : Zoro has Baratie promise, so he can't lose. His entire selling point is about how cool he is in battle.
Thus, using Zoro as hypetool is counterproductive in the bigger picture.
3. Jinbe : This is Jinbe's first arc establishing as a nakama. His debut arc winning some Flying Six, then suddenly he look very bad on the next arc, it will be a letdown.
Thus, using Jinbe can't be done because he is new.
4. The rest of SH is too weak. Thus they can't be used.
5. Sanji's character focus is his writing. He is being related to everything by Oda.
Like, even when Dressrosa shouldn't be his arc, he got intelligence moment and turns Viola.
Thus, Sanji is used because Oda simply wants to shove him in everything. Like everything. For this, Oda wants to tie Sanji to Five Elders.
e.g. :
- Science as a system should be exclusive to Franky, yet Oda shoves Sanji too to this.
- Revo as a faction should be exclusive link for Robin (beside Luffy), yet Oda somehow makes Iva's people to be cook.
- Royalty as a class should be exclusive status for Vivi, yet Oda somehow reveals that Sanji is a prince.