A linguistic detail I've found fascinating on re-read is that Shanks' crew call him kashira rather than captain. Other crews usually call their leader some version of 船長 (senchou) or キャプテン (kyaputen) - both translated "captain". Some pirate leaders have fleets, and use some version of "commander", but kashira is particularly un-nautical? It's used for organization bosses. Bandits might have a kashira, but not pirates! And it's not just his crew: the first time he's introduced in present time here, the manga itself calls him ookashira - big boss, or leader of a large group.
I'm wondering if somehow for Shanks, there's only ever going to be one "captain". To the point where he's gotten his crew used to having a kashira rather than a senchou.
Or, you know. He's got a side gig to pirating that everyone but the reader knows about, and that's what his title refers to! It's One Piece; I don't so much have theories and analysis as vague thoughts in random directions.