There are opinions. Everyone can have them. "I like this character, I want to see more of them." Great, absolutely no one can argue that, enjoy. Wapol is one of my favorite characters, I look forward to him being at the reverie.
There are theories. These need to be backed by facts and proof. "I think this character will join because…." is inviting discussion, and then debunking or proving.
We come down so hard on it because the story itself, Oda's work as an author and all the setup and technique he uses is being ignored and discarded when you use the "It's One Piece, anything can happen!" method of theorizing.
This is an outright falsehood born from people just not paying attention.
The actions of a fight scene, brand new characters, applications of their quirks and abilities, and outright secrets Oda has intentionally hidden? Those things are random and new and can't be thought about in advance because that is the stuff only Oda knows.
But or all the characters already in place? The story elements already established? The structure is incredibly solid and open for very accurate advance analysis and theory. For instance we figured out Momo was son of the shogun, that Kaidou had ravaged Wano, and probably has a dragon zoan... three years ago all the way back on Punk Hazard. People figured on Buggy being a new warlord years before it was confirmed. We figured on Lola being connected to Big Mom the instant we found out one of the Emporers was a woman like 6 years ago. We assumed Big Mom was going to have a sort of Alice in Wonderland theme based on Pekmons being a lion turtle and Tamago being what he is. Monkey King figured out the basic end plot of the series nearly 10 years ago, long before Rayleigh and WHitebeard revealed pretty much the same things coming. Greg has probably cracked some major series end secrets already.
Heck, even Naruto shared some structure things and we deduced, from minor one panel evidence in the story,years in advance, that Itachi was secretly good, that Tobi was Obito, and that there would be a 10 tailed demon in the moon, among other things. Accurate prediction and theory, not just random guessing and hope.
Story structure is there, and in Oda's case it is super, SUPER solid. A little observation and study goes a long way.
If you want a series where "anything can happen!" and its "totally unpredictable, the author can do anything he wants!" then go read Bleach, where the author has blatantly admitted to not knowing what is coming even two weeks ahead, throwing out random new character designs when he hits writer's block, and he is totally pulling his "plot" out of thin air constantly.
So yes
If you like a character, great! If you want to debate "I think this specific thing will happen because..." be it crew mate, upcoming arc, someone living or dying, or whatever else, prepare to have that reason questioned and debated.
It's not a personal attack to have a bad theory ripped apart. It's to help everyone grow in appreciation for storytelling, not just in OP, but all stories.