Which might be true, but it's also not that relevant. The defensive spectrum is not an academic argument about the difficulty of various positions. It's a pragmatic recognition of the size of the pool of available players at various positions (which affects the amount of offense that can be reasonably expected), and the history-of-baseball-long path of transitioning from a more difficult position to a less difficult one. Argue the difficulty of third base all you want -- and I agree, it is quite difficult to play well -- but over baseball history, lots of major-leaguers have moved from shortstop to third base, permanently or temporarily; they hardly ever move from third base to shortstop. The "why" isn't important in context, it just is.