The battle of the Onigashima has finally concluded with Luffy’s victory over Kaido. The Alliance members celebrate the news. Yamato meets the rest of the Straw Hat Pirates for the first time and reiterates that she will join the crew.
It’s already been established that Yamato’s dream is to go out on the sea and have freedom and adventures. She has never been portrayed as wrong or foolish for wanting this (unlike her obsession with becoming Oden), and it’s pretty clear this is what Oda wants us to be emotionally invested in. We want Yamato to succeed in winning freedom from her father and going out to sea. Having her just stay behind in Wano would essentially rob the emotional catharsis of that event.
I’ve seen several people assume that Yamato only wants to become a pirate because Oden was one. Because it seems her character arc will be learning to stop imitating Oden and be her own person, I’ve seen several people assume that her dreams of adventure on the seas are a part of this and she should drop them. I think this is a fundamental misunderstanding of Yamato’s desire to be Oden. Yamato doesn’t want to have adventures because that’s what Oden did, she wants to be Oden because he had adventures. Oden represents to Yamato the ideals of freedom and honour she values. In the Ace flashback her excitement over learning about the Worst Generation and her admiration of Luffy for having the same dream as Gol D. Roger also shows she has a general interest in pirate adventures beyond just Oden’s.
Wano doesn’t need a guardian
This is the big one. Okay, so here’s a good example. Fishman Island. That place was so weak it was nearly conquered by freaking Hordy Jones of all people. The only real super-strong person there to defend it was Jinbe. When Jinbe was asked to join the Straw Hats at the end, nobody said “but Fishman Island needs its strongest warrior to protect it”. Now Wano has a lot more powerful fighters than Fishman Island. Fishman Island was threatened by Hordy Jones, who was ultimately a weakling in the grand scheme of things. Wano was conquered by Kaido, literally the World’s Strongest Creature, and even he had to be underhanded about it by having Orochi invite him in and tricking Oden into wasting his time dancing while the Beast Pirates grew in strength.
In addition to all the strong fighters who are already native to Wano, we have to keep in mind that Wano will probably become Luffy’s territory and be under his protection, just like he said he would do with Fishman Island. So if Yamato wants to protect Wano, she could just do it as part of the Straw Hat Pirates.