For a long time, One Piece manga readers have been trying to figure out whether Yamato, the son of Kaidō, would be welcomed aboard the Thousand Sunny. Ever since the idea was mused a couple of years back, there's been talk and speculation of what it would be like if she ended up becoming a member of the Straw Hat Pirates. The way the story was being written, it seemed like she might join, but there were no guarantees.
Chapter 1057, "The End," finally confirmed that Yamato would not be getting on Luffy's ship. Reactions to this confirmation are mixed; some fans are having trouble coming to terms with it, while others are expected to dance with joy. There were all sorts of arguments for and against Yamato joining the crew, and this latest chapter has made all of them more relevant.
The idea of Yamato boarding the Sunny was first put forth in Chapter 985, "New Onigashima Project." The Oni Princess explained to Luffy that she wanted to board his ship and leave Wano just like her idol, Kozuki Oden. This chapter was released on July 20, 2020, so fans have had plenty of time to think about it.
This wasn't just something that was said in the spur of the moment, either. She reasserted her intent to join the crew in Chapter 1051, "Shogun of Wano, Kozuki Momonosuke." Jimbei noted that Luffy still needed to give his approval, though there was little reason to believe that he wouldn't.
It was also supposed to be a part of Yamato's character arc that she eventually leaves Wano and explores the open seas. She had essentially spent her entire life living as a prisoner on Onigashima because she sided with the people of Wano over her father's Beast Pirates; she even had to wear explosive shackles until Luffy broke them off. Now that Kaidō is out of the picture, she has the freedom to leave the country as she always wanted.
Not only does Yamato want to leave Wano, but her friends on Wano want her to leave. In Chapter 1055, "The New Era," Momonosuke insisted on fighting Ryokugyū alone so that Yamato could pursue her dream of leaving the country; if he could protect Wano without her help, then she wouldn't have any more excuses to stay.
Joining the Straw Hats would also have helped Yamato in her efforts to follow in Oden's footsteps. Going with Luffy to find the One Piece would be like her idol going with Roger to Laugh Tale. In this sense, it would be poetic if Yamato were there with the Straw Hats for that last leg of their journey.
As far as being a fit for the crew, Yamato has plenty of the qualifications. She has an amicable personality, a childlike sense of wonder to rival the captain's, and a great deal of ambition. She's also proven herself to be more than strong enough as a combatant; some even believe her to be comparable to the heavy hitters of the crew like Zoro and Sanji. She's also one of the few people Luffy's met who knew Ace well and personally; having her on the crew would be an opportunity to explore this aspect of her and Luffy's personalities. Luffy mentioned that he wanted about ten crewmates, so there should be room for her.
Of course, for all the reasons there were to be eager for Yamato to board the Sunny, there were also plenty of reasons to be against it. For one thing, as far as introducing new crew members is concerned, it might be a little too early. After all, Jimbei was only formally welcomed as a member of the crew with the start of the Raid on Onigashima; this was after years of alluding to him joining the crew ahead of time. Having Yamato join after that might come off as too soon, even if it's been two years in real time.
Some less patient naysayers will bring up Yamato's insistence on being Oden as a point of contention. At times, it could come off as excessive, as though it was her only character trait. This is to say nothing of the resulting debate surrounding the Oni Princess' gender identity. Many fans grew weary of it in Wano, where it was most appropriate, so having her continue with it for the rest of the series could prove unbearable.
On that same note, Yamato doesn't really have anywhere to go as a character. If all she wants to do is play out Oden's journey, that doesn't make for a compelling character. There might be some story potential for her to learn to be more of her own person, but whether that will actually be explored remains to be seen. Considering how she's staying in Wano so she can continue on Oden's path, that's probably not where her story is going.
Even if Yamato were to join the Straw Hats now, there's not enough time in the series to develop her or get invested in her as a crew member. The series is currently in its final saga and is expected to end within a few years. In this regard, if anyone ends up in the final slot, an older character who's been properly developed would be far more suitable.
To top things off, nobody's certain what role Yamato would even have if she were to join the Straw Hats. Some believe she could be a journalist like Oden, but, again, there's not enough time left in the series for that role to be relevant. Unless she interviews individual crew members about their past exploits, she won't have much to write.
Whether Yamato should have become a Straw Hat or not, what really got to people was how the whole situation was handled. From Chapter 985 right up until Chapter 1056, "Cross Guild," the subject was technically left ambiguous. Nothing confirmed that Yamato would join the crew, but nobody really said she wasn't welcome aboard, either. Technically, it was always left just open enough to interpretation that things could have gone either way.
However, the way the story was written really seemed to point to Yamato joining the crew. Between her apparent certainty she'd be welcomed aboard and all her friendly interactions with Luffy, the story seemed to be easing audiences into the idea of a new shipmate. To backtrack on two years of that would come off as even more forced than if she had actually joined.
Unfortunately for the fandom, that's precisely what happened. Yamato decided she would travel around Wano before making her way out to sea; she came to this conclusion in an off-screen conversation with the Straw Hats. With this simple decision, she's gone from a potential crew member to just another person they happened to meet in Wano.
For as much as the subject was teased, having Yamato appear to change her mind at the last possible minute comes off as a bad retcon. Whatever the reason for this twist, it doesn't seem like her future with Luffy should have been teased for two years just for her to be left behind like this. Both lovers and haters of Yamato share the sentiment that this was an ill-conceived way to handle this character.
Of course, One Piece is not without the cruel mercy of hope. According to Luffy, he'd be willing to pick up Yamato or any of his friends from Wano if they ever wanted to become pirates. However, given how rare it is for the Straw Hats to revisit islands, not many fans are ready or willing to be fooled twice. Also, for anyone hoping that Carrot will join the crew, that's yet to be disconfirmed.