Enjoyed your explanation and agreed/understood most of it but cant quite see why aspect ratio of planing surface affects ability of a board (or any sailing craft) to sail deep down wind.A big powerfull sail is a must and in sufficient wind /control it is possible for craft to approach planing on a dead run. A kiter (perhaps when overpowered) can plane in direction of wind.High performance dinghies can for short periods maintain planing on a run.
Neither of these craft have either high aspect planing surfaces or particularly efficient ones; but in both cases control of the power is still possible whilst on a run.Not sure thats the case with boards.Launch a kite and in sufficient wind almost any board would plane on a run !
Down wind the board is a platform to utilise available power; upwind it is a sailing craft ! ???