Oda really screwed up when most people now care more about the side characters and the world than the main cast. That might also be because too many readers latched onto One Piece after Naruto ended, rather than being fans from before the timeskip. This is exactly why splitting the crew up at Sabaody was such a bad idea — it completely killed any sense of focus or momentum for the Straw Hats outside of Luffy.
Sure, Whole Cake Island gave Sanji some spotlight, but the key emotional thread — destroying his final connection to his past — should’ve ended with a proper clash against Germa. Instead, it was halfheartedly resolved in Wano against some random ex-lab partner of his dad's who copied his family’s abilities rather than confronting his actual family. There’s also some implication that Brook might finally become relevant again with Gunko, but we’ll see whether Oda actually follows through or drops the ball yet again.
At the very least, Oda should’ve kept part of the crew with Luffy after Sabaody and had them infiltrate Impel Down alongside him (or even have them be imprisoned there, forcing Luffy to rescue them and Ace together). The Straw Hats with the most combat ability — Zoro, Sanji, Brook, and maybe Robin and Usopp — could’ve gone with Luffy and Jinbei to Marineford. Zoro’s injuries from Thriller Bark could’ve easily been fixed with Ivankov’s hormone nonsense, and as for Robin and Usopp being “too weak” to fight, they could’ve simply stayed on the sidelines. If a clown like Buggy and a haki-less Luffy could survive Marineford, then plot armor could’ve easily saved them too. Nami can still be shipped off to Weatheria, and Franky to Karakuri and Chopper to the giant bird kingdom for their specialized skill training.
After Marineford, they others could have been sent to Rusukaina with Luffy to train under Rayleigh in haki. That way, Zoro wouldn’t have to grovel to Mihawk and could continue mastering his swordsmanship independently (even if it meant losing some cute Perona moments). Sanji wouldn’t have wasted two years on an island of okama predators only to learn how to fly and never learn a thing about newkama kenpo — and that “attack cuisine” has been practically useless outside of two brief moments after a battle. Usopp could’ve learned real haki instead of relying on those magic plants, or they could’ve simply made Boin Archipelago’s plot plants native to Rusukaina instead. Brook could’ve trained his haki and mastered his fruit through meditation — his world idol subplot has added nothing to his character, and Gunko could have recognized him anyway without the concert nonsense, since she didn't even realize he was the same Brook from her past until after Colon started screaming for his dad.
Robin, meanwhile, could’ve trained with the women of Amazon Lily and learned haki properly, since she gained nothing of real value from the Revolutionaries. All she got was some fishman karate from Koala that’s barely been relevant.
In short, Luffy and most of his nakama could’ve remained central to the story — developing alongside him — without letting the focus shift entirely to side characters, Joy Boy, or Nika nonsense like it has since the timeskip. Because all that's done is make newer audiences have almost zero interest in most of the Straw Hats and see them all as clowns irrelavent to their own story. And any few remaining Usopp fans have gone dead silent after Elbaf proved to be a no-show for him.