Momo's character runs parallel to his father and is in line with the main themes of the arc.
Oda went to absurd levels to wank Oden as some uber chad who could do anything and everything by himself. He turned down sharing any form of responsibility and took on everything on his own. From the very beginning when he refused to allow Kin emon to share blame for the mountain boar, to refusing to accept help from Roger and WB to deal with Kaido, to his execution when he lifted up those that were supposed to protect him from harm. He did everything alone.
An aspect of Momo's character arc is about him growing to accept the help of others and working with them.
At first he had the same mentality as his father. Refusing to accept help or pity.
But then by the end of the arc he'd already grown a bit. He reluctantly accepts Sanji's meal and joins in with the feast.
Then comes his first big character moment. The forming of the alliance where he finally shouts out his inner most desire.
Soon after Momo commanded Zunesha to help him defeat Jack.
Then you have Momo inspiring the Udon prisoners and reigniting their will to fight with him alongside with Luffy.
When he's captured, Momo has a lapse and feels he needs to do things on his own again and that he doesn't want to be a burden, but Luffy reminds him that since they're friends they'll find a way to help him.
Then of course you had all the Yamao stuff. Accepting the help of his greatest enemy's daughter and developing a close bond with her.
And finally we've got him asking Shinobu to help him to age up so that he can then help save Wano, not on his own, but with Luffy at his side.
His whole character journey has literally been about him learning to work with others and develop out of that loner, stand-off nature he originally had. It's therefore absolutely no coincidence that the main goal of the arc is to open Wano's borders.
Wano's borders need to be opened so that Wano can work with other people and nations. Wano as a country is just like Oden was. Isolated, detached from all external support and just doing its own thing. The borders of the country need to be opened so that the country and the entire world can begin to prepare for their common goal. The battle against the world government.
EoS is closer than you think. For a war that's big enough in scale for the climax of a series like OP, the preparation needs to start early on. The allied nations, tribes etc. all need to come together at some point. But how? The WG is already preparing its next "great cleansing". The allied side needs to also start preparing. It's been mentioned time and time again that Wano is a natural fortress. Again, it's unlikely this is a coincidence. Wano Kuni is built for war.
Momo, as the shogun of Wano, as a Kozuki, as a man with the voice of all things, is the perfect person for the job of reconnecting the ancient world and asking for allied nations' aid. His entire character arc has been building him up to do just that. He's simply too important of a character to just remain in Wano when so much craziness is going on in their world right now. Momo
has to leave and the odds are he's once again gonna ask for the aid of his newfound chum Yamao to help him do it.