Shanks being a "rat" was the only interesting aspect about an otherwise poorly written, generic gary stu with zero depth. With Shamrock taking that away, the ideas of shanks being some kind of "secret final villain" or serving any significant antagonistic role have been flushed down the toilet. Even shanks fans themselves started to realise how flat the character was, which is why they are the ones that have been pushing all these outlandish shanks antagonist theories. They wanted to put him in the big boy final antagonist group. They deluded themselves in to thinking shanks was some kind of complex, nuanced, mysterious 200IQ character who controlling the whole one piece world. Shanks purpose is to simply be "cool" and give children and pea brain adults a hit of dopamine when they see nice guy win against not nice guy. He was never going to be anything more than that.
Not only is shanks character affected, so has his potential power ceiling. Shanks is not getting that endgame antagonist boost. He will be taken out before the endgame begins to pave the way for the true endgame players. The "good guy" that will be scaled to the endgame villains is Dragon, not Shanks. Oda's recent glazing of shanks along with the movie is an indicator of his impending downfall. Now Oda just needs to get his celestial backstory out through shamrock/garling lore. When that's out, Shanks will be dispatched once oda decides to move the story forwards.
There's plenty to hate about an evil twin being thrown into the story or whatever, but like 90% of people whining about it is just the stock of *Projecting a hypothetical/made up version of a character onto the real version of them, and expecting it to come true* crashing for people. WOAT case of it too cause it's the most obvious good guy character in the world, and people deluded themselves into thinking he was on some secret evil double agent shit, at the center of a high level conspiracy
When it comes to Shanks' depth as a character outside of that, there's plenty of room for Oda to write a story about Shanks that
is compelling without having to make him Aizen. He already has interesting details being revealed recently, like how there's someone in his life that seriously judged him over his decision to not follow Roger's legacy- obviously, "it's Buggy haha" but this is the only person qualified to judge his character as a peer at that time, literally raised as his brother, and he thinks of him as a let down. It genuinely does indicate something for his character going forward, and there's clear potential there for Shanks' story to build on top of pieces like that. Whether it's going to be about how he didn't live up to other people's expectations to be Roger's successor, or how he was born too late to be Roger and yet too early to be his successor (Luffy), or how he isn't the man people want him to be, etc.
Obviously, that's also entirely speculative, and even if I end up right, I'm sure I'll be wrong on the exact details too- so my point is just that there's multiple avenues for the story to actually write something around who he is without him having to be written like someone's edgy OC. And at least in this case, there's something in the text supporting the proposed writing/characterization. He's also a character who hasn't been the focus of an entire arc just yet either, so obviously his character has yet to be fully explored, while also being someone that's been on Oda's mind for like 20 years, there's gotta be something there saved for him, even if it's not that deep in the end.
Also important- he's not the MC. He makes a lot of money for Shuiesha, Toei, and Oda, so he gets some love. He's the MC's goal and full circle moment, so he gets some love. He communicates basic ideas of what a decent person is to a young and impressionable audience, so he gets some love. But he's just not the MC. He doesn't have to be the strongest in world history, or be the greatest character ever. Shanks fans that act like 'this
has to be true', cause people treat agendas like sports teams that they just bet real life money on or something, are doing him a greater disservice by setting him up for future slander in reality. Truthfully, he just has to play his role in the story and be strong enough for the MC's journey to be meaningful with him as a goal. People reading a story arc about that in real time will just admit "wow I can't believe it's finally happening man 25+ years, 1100+ chapters later, it's all coming full circle, kinda cool". Nobody, besides people reading the manga on 800% zoom and a ruler taped onto the screen, trying to decipher the exact amount of damage that a punch did based on how much dust particles are in the air, is gonna be sitting there thinking about how Shanks should have been "Warcury+ level" or "Joyboy level" or whatever lmao