1. Duval who wanted to kill Sanji and the entire crew eventually became so loyal to Sanji that he ended up guarding Luffy’s ship for two damn years. Duval is an incredibly loyal follower, he’s like Sanji’s Kinemon at this point, he would easily sacrifice his life for Sanji.
2. Gin the guy who was famous for his coldblooded nature ended up sacrificing his life for Sanji. That’s a very strong Conqueror’s Haki right there! This dude is so loyal to Sanji that he gave his gas mask to Sanji.
3. The G3 Marines ended up following Sanji just like many female characters like Viola or Pudding etc. You get the point…
Oda has actually introduced characters who would be Sanji and Zoro’s right and left hand men if they were captains.
Gin would have been Sanji’s right hand man and Duval would have been his left hand man.
And the same for Zoro with his brothers Johnny and Yosaku…
So, in conclusion: whether you like it or not, whether you hate it or not, Sanji will get Conqueror’s Haki. Sooner or later, one way or another, he will awaken his Conqueror’s Haki and that’s inevitable. And Luffy’s crew will surpass Rocks’ crew.
*Theory by Master Of Haki