The question that needs to be asked is how is Yamato going to fill the void of information currently existing in Momonosuke? Sure Yamato has context and had 20 years to familiarise himself with the contents of Oden's journal but the aspect Momonosuke is stressing over are in regards to the page that Oden tore out of his journal. The page that related to what Oden found on Laugh Tale with the Roger Pirates.
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Yamato cannot help Momonosuke fill that gap in the state he is in as he does not currently know what Oden discovered on Laugh Tale. Rather, that scene which some believe supports the possibility that Yamato will remain in Wano actually entrenches the likelihood that Yamato will travel with the Straw Hat Pirates. Ask yourself, will Yamato staying in Wano solve that particular issue for Momonosuke? Without the information Momonosuke is seeking, it would not. And the only way Momonosuke will fill that void of information is knowing what Oden found at the end of his journey with the Roger Pirates (the "Dawn" is an end story plot thread). Now, the important part is who Momonosuke is confiding in. It is not Luffy or Kin'emon or Robin (who Momonosuke knew longer) but Yamato. Momonosuke is sharing their frustration with Yamato. Information that can only be found on Laugh Tale. The scene in question doesn't keep Yamato in Wano but attempts to push Yamato toward the secrets on Laugh Tale so that when Yamato returns to Wano, he will be able to share it with Momonosuke. Luffy will not be the one to convey this information to Momonosuke as Luffy doesn't care about such information. It will be Yamato who Momonosuke confided in. A character who Momonosuke has recently gotten close to. A character whose interest aligns with Momonosuke's - Joy Boy, Wano's survival, and the secrets Oden discovered on Laugh Tale.
What Yamato got from that scene isn't Momonosuke not being ready to carry the burden but that Momonosuke also desires to know what is on Laugh Tale. If Yamato identified that Momonosuke was not ready to carry the burden, he would not be expressing to Momonosuke that he is the only solution to Wano's complete victory. Yamato BELIEVES in Momonosuke and believes he is ready to carry the burden of leadership as Shogun. Yamato believes Momonosuke can make flame clouds to keep Onigashima afloat which is akin to Momonosuke carrying the future of Wano and the World on his back. If Yamato did not believe Momonosuke was ready, he would currently not be requesting this from Momonosuke.
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Momonosuke may not believe in Momonosuke but Yamato definitely believes Momonosuke is ready (and the above scene takes place AFTER Momonosuke confides his frustration in Yamato). Also once Momonosuke is able to produce sufficient flame clouds to carry Onigashima, I don't believe Yamato would have anything else to teach Momonosuke in regards to the Artificial Seiryu Devil Fruit.
Momonosuke won't receive an answer to what the torn page from Oden's journal contained at the conclusion of this arc but Momonosuke will arrive at a realisation of what direction to take Wano in. Luffy (Joy Boy) will be integral in this realisation. This is why Oda has Momonosuke witness Luffy revive into a majestic form and fight Kaido as an equal. Momonosuke needs to know that Luffy is capable of changing things and that it is okay to place his (and Wano's) hopes in Luffy.
@Syphin:
Oda has intimately tied Wano's fate and story with that of Luffy and the Straw Hat Pirates. This is why Oda has likened Luffy to a Guardian Deity of Wano through Hyogoro's comments. The possible fallacy in certain lines of thinking is believing that being a Guardian Deity of Wano equates to remaining in Wano but that is not the case. Wano's story is expanding beyond the lands of Wano Kuni toward the rest of the world. Oda has time and time again hit us with the reality that Momonosuke will be the one to guide the World to the DAWN. Not only Wano but the whole WORLD. Think about it, what does this mean? If Wano's relevance is encapsulating the rest of the world, does this not also mean that their influence and story are also extending beyond Wano?
Oda has clearly made it a point that beyond the Wano Kuni Arc, the role Wano takes will be completely different. The borders of Wano Kuni will open and the characteristics that defined them will be transformed. Change, growth and liberation are themes of Wano Kuni's story, especially in regards to Momonosuke. As the future Shogun, Momonosuke will bring Wano into the future and connect it to the rest of the world. Momonosuke will evolve Wano from what it was and become a leader second to none - as his name symbolises. This is why the current story of Momonosuke is dealing with him figuring out how to move Wano Kuni forward. Momonosuke believes keeping Wano's borders closed may be the right thing and that surrendering to Kaido would be the path forward. Relative to the story that line of thinking is wrong but this is where Momonosuke grows into a leader Wano Kuni and the world need. He is asking himself and Yamato these questions and thanks to the guidance Yamato is providing him, he is starting to understand what answering them could mean. Momonosuke knows he is important but doesn't exactly understand why. Momonosuke is seeking to clarify how the pieces of knowledge he has fits together.
Once Momonosuke comes to this realisation, he will understand that the battle he is fighting doesn't just involve Wano but the rest of the World. What is in balance isn't just the lives of the people of Wano. And thanks to reading Oden's journal and speaking with Zunesha, he now recognises how important Joy Boy is to this future. Momonosuke will realise that in order to reach the future his father, mother and ancestors fought for (including closing the Wano borders 800 years), Luffy/Joy Boy is necessary. Sure Yamato may turn out to be the Guardian Deity of Wano but with Luffy embodying the future of Wano as Joy Boy, sailing and fighting alongside the Straw Hat Pirates is arguably synonymous with fulfilling the role of Guardian Deity of Wano. Plus let's not forget that after Joy Boy's unfulfilled mission is complete, Yamato will return home (Wano Kuni).
Rather than the Guardian Deity of Wano thread being a limiting factor that keeps Yamato in Wano it in fact serves to support Yamato helping Luffy.
@Syphin:
After witnessing the sheer madness unfolding before them, both Momonosuke and Yamato should now realise who Luffy is and how necessary his presence is to the arrival of the Dawn. Yamato suspected it and believed in it. Now he is getting the confirmation that Luffy truly is the individual Oden had been waiting on. Momonosuke as well is coming to a realisation of how special Luffy is. By being spectator to the battle Luffy is having against the world strongest creature, Momonosuke and Yamato will arrive at a path that they will realise would need to be traversed on. Momonosuke had questions before about what decisions he should make regarding Wano. The battle Luffy is having against Kaido should provide Momonosuke a direction he needs to head toward. Luffy is important to Wano's dreams (that Oden and the past Kozuki clan members held) and opening Wano's borders to support him will become a decision Momonosuke will find himself needing to make. Supporting Luffy will be the result of what both Momonosuke and Yamato are currently seeing. As for Kaido, he too will come to realise the freedom on display before him is the embodiment of “Joy Boy”.
It is also becoming clear why Oda spent so much time developing the bond between Yamato and Momonosuke. If not for that time spent, it would not be believable for Momonosuke to rely and lean on Yamato as much as he currently doing. Going forward Momonosuke will understand Luffy's importance and how intertwined Luffy's journey is to Wano's survival and prosperity. Momonosuke knows Yamato admires Luffy and when Momonosuke learns that Yamato plans to travel with Luffy, he will have no reason to challenge it. Rather, Momonosuke would be encouraging Yamato to help Luffy as that will in turn be of benefit to Wano (the best course of action). Momonosuke will want to know what his father discovered during his journey and he will rely on Yamato to relay that information through the journey Yamato will have with the Straw Hat Pirates.
As for those feeling the Yamato support is too strong, there is genuine reason for that. He is currently the only character with momentum in regards to being a potential Straw Hat Pirate and with each passing chapter, the likelihood of Yamato remaining in Wano and not traveling with Luffy decreases. My theories on Yamato (+Monet, Kaido and other characters/developments) have been substantiated by the manga and possible interpretations of the flow of the story. I do not mind being wrong but I do feel I bring value to the discussions.