[Kurona]

That was a point Decade had, though -- one made absolutely explicit in his Movie War with W. That he was a wandering traveller with no story of his own; someone who was only haphazardly shoved into the stories of others. His entire role and identity changing with each world was largely played for laughs, but it still very much illuminated the point that that was because he had nothing of his own and was only made more pressing when his companions didn't change alongside him.

... granted, it could have stood to be better. Tsukasa and Natsumi for me are characters that absolutely save a season that was otherwise a bit too confusing and perhaps thought a bit too highly of itself and its concept; but said concept being a metacontextual look at what exactly Kamen Rider was at the time still manages to be an intriguing one.

Gaim and Zi-O take their predecessors' power as armour. Ghost inherits their souls. Drive uses their skills and Ex-Aid proves once again that he really likes video games. But Decade only has a bootlegged version of the literal rider form itself rather than any derivation or anything to make it truly his; hell, the Decadriver itself is modeled after previous belts and is nothing more than a proxy to bypass the abilities of other drivers.

Decade simply uses others' powers without what made them special; has nothing of his own that makes them special; and is himself not special. And that's the point.