There are a lot of technical differences between SS, 2B, and 3B. I think they play very differently. 3B is all reaction and athleticism. 2B is range, finesse, footwork. SS is everything - range, finesse, footwork, arm strength, rhythm, consistency.
There are a wealth of players that are good at 3B, but bad at 2B (or vice versa) despite the fact that the positions are equally important. Brett Lawrie is a good example.
Compare to the OF where every position entails largely the same thing - getting a read on a ball and running it down. The only real technical differences are the importance of a good arm, the fact that CF is simply bigger, and the fact that the angle changes.
The amount of LF or RF that could play CF and not even look that bad to the naked eye is pretty high. Remember when Bautista played a bunch of games in CF a couple of years ago? You might not even remember because it was rather uneventful. Look at Matt Kemp - a LF patrolling CF for years. He even won two undeserved gold gloves doing it! That's how easy it was for him to fake it.
Most of the top 3B or 2B would look out of place almost immediately at SS. If Toronto put Donaldson there for a whole game, or a whole series, you'd probably see a bunch of glaring misplays that a typical MLB shortstop makes.
I really don't think the disparate demands of the sets of positions are congruent at all.