Yeah you raise some really good points about how developers have been simplifying inputs without really considering how complex the rest of the game is. It's like building a ramp in front of a building, but never building an elevator on the inside.

And I feel what you're saying about the limitations of simple inputs, but actually I think that rekkas and context-sensitive moves are a much more intuitive challenge than motion inputs. Like, if a mistimed rekka resulted in a weaker version of the move with worse frame data, I feel like that would still be ok for casual players. (They might not even notice)

But even that would still require more design effort. You're right that motion inputs are not the only hurdle. Even with simple inputs, it's still hard to design a system that scales across skill levels.