It has nothing to do with Power Levels
Luffy is part of multiple Groups
He forms a Trio with Ace & Sabo
He forms a Trio with Zoro & Nami
He forms a Trio with Law & Kid
Garp & Dragon
Current Emperors
Zoro, Nami & Usopp represent his Power, Wealth & Fame (Reference to First Panel)
Each has a Significance & specific relation to a certain Sub-Plot in the Story
And the Trio he forms with Zoro & Sanji is one of the Major Ones
If you go check entire Manga from start to finish, there is no "Unique & Exclusive" Moment between Zoro & Sanji that is related to Luffy except "Nothing Happened" Moment, and that was just a Foreshadowing, what's special about that Encounter? It happened "After" Main Villain was Defeated, something that never happened in any Arc
So imo, Oda created this Monster Trio which later he further hinted at it's Importance with "Wings" Statement cuz at the End Zoro/Sanji are gonna do something "Unique" for Luffy & no one else in verse can replicate
It's like Zoro & Sanji are fated to do one final Service for Luffy that they were built to do, only them. And if you check their Skills, we know that Zoro is on Path of becoming Best Swordsman Ever, while Sanji will Reach his Max Flame/Body/Emotions Potential
In other words, at the end of One Piece, there are gonna be Two Obstacles standing in Luffy's Way (And it's clear that this moment will happen "After" Final War & Final Villain is Defeated):
1. One of these Obstacles is something that no one can ever Cut except Zoro (Explaining why his Dream was important for Luffy's Journey)
2. Second Obstacle is something that no one can Endure except Sanji !!
And since we aren't talking about Power Levels here, it means the First Obstacle is something that cannot be touched by anyone except Zoro, meaning it's beyond what anyone faced before (Not Haki, not a Fighter, not DFs, not Swords, something Unique). And it also means that Second Obstacle is something that Sanji specializes in enduring better than anyone in existence, meaning it's "Extreme Heat/Flames"
The Answers are simple imo:
1. Death Itself is gonna come for Luffy (Similar to how Kuma targeted Luffy)
2. The Sun (Or an Equivalent Flaming Place) needs to be approached
That's where Zoro/Sanji Final Roles will become clear & show why they are indeed Wings of Luffy & portrayed as his Right Hand Men cuz only these Two can save Luffy from Death itself & approach The Sun (Only Place that no one ever reached)