Please don't lose hope yet, I didn't.
I'll tell you why.
Whatever GB did to King, we never even saw him shrink. But even if he did, once GB has left, all he left behind him was a natural environment. King can survive any natural environment, so I don't believe he's dead, even if GB and many fans may think so.
Also, he knows Yamato knows about Joy Boy, and Yamato stayed in Wano as well, with a promise from Luffy to come pick her up when it's time. Meanwhile, as Yamato wants to follow Oden's footsteps, her "new scabbards" surely will gather around her and I'm sure King will be one of them.
If he's too physically big for the Sunny, he can follow them with his own ship, I'm sure he has one.
And if, by any chance, Oda forgets about King and doesn't make him turn to Luffy's side eventually, there's always
my fanfic which gives Wano a different ending, one I think you'll like better.
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He was a good boy, an innocent boy even. Still, he lived for more than 30 years under Kaido's influence. I'm sure he killed many who didn't deserve it under Kaido's command. However, he was not the sadist people thought him to be.
Out of loyalty to Kaido and thankfulness for saving his life, he gave up his own dream and commited to fulfill Kaido's. Now that Kaido's dead, he has the opportunity to follow his own dream again, which of course popped into his mind the moment he was defeated.