Yes, it is a fan request but Oda could've shown them actually fighting it out, or even Zoro triumphant. He didnt, he showed them both imagining each other winning. I get that the cover page isnt proof of them being equals, but its proof that Oda is portraying a rivalry between them.
I do agree that the new world has been a disappointment in the way Sanji has been portrayed. Its true for all the strawhats except Luffy, but its worst for Sanji.
Your reasoning is poor . Lost to everyone, really? So would you say Zoro lost to Kyoshiro? No you wouldnt, cos that wasnt a complete battle.
Sanji vs Vergo, that was one moment and an incomplete battle. Sanji beat Yonji, would've gotten away beating Judge who is the strongest.
Against Niji, he was warned if he fought back, they'll keep Zeff which is why he stopped. Not a battle again.