I find that an disingenous argument to make.
Of course you can take a plot seed and move it around between characters. But that ignores that each character—if they are well-written and well-developed—will react to and interact with the story in very different ways. And not every story fits into each and every phase of a character's life in the same way.
If we take Burnside as an example, it'd have worked for Steph, and possibly even Cass, because the setting wanted a character with limited "real-world" experience but still some "cape" experience. It could have worked for Babs, but that would have required setting it before TKJ, and possibly also reworking her entire background.