Equilibrium models cannot properly describe the physical phenomena at hand –– suitable craft going downwind ––, because in the real world, wind speed is not constant, nor does it have a perfectly stable direction.
When you simplify the situation enough so you can apply equilibrium physics, you're essentially discussing how a toy on a rail in a perfect wind tunnel behaves. I don't think that is useful; it's not what happens in real life.
Consider a wind pattern where you have some base wind speed
X, with a roughly sinusoidal component on top (although the exact shape or frequency does not matter, as long as the changes occur in relatively short timeframes, say on the scale of seconds, and they're symmetric, not affecting the average wind speed –– this is more or less quite typical wind behaviour). For now, let's assume it stays in the same direction.
Instead of a fabric sail, let's imagine you have wanes that act like a one-way valve: when the craft velocity is below wind speed, the wanes catch the wind, and when the craft velocity is above wind speed, the wanes let the wind pass through mostly unhindered. It won't be perfect, but all we're looking for here is asymmetry.
Even on a fixed straight track in a wind tunnel, that craft will reach a speed that exceeds the average wind speed. If there were no losses, it would reach the maximum repeated wind speed. Some, perhaps most of the time, it is exceeding the wind speed, only getting additional power from the gusts, "peaks".
It is quite analogous to the behaviour of charge in a capacitor.
The second thing is the wind direction. When sailing on sea, you don't get the best efficiency by catching the wind like a parachute: you get best speed by using your sail like the wing of an airplane (very similar flow profile, too). You can make a pretty good sailboat by making a vertical wing (like the ones on aeroplanes, but vertical) that you can rotate around its vertical axis. Now, I do not claim to know or understand
fluid dynamics (physics sub-field that deals with the flow of liquids and gases) nor sailing, but it should be obvious that wind direction, even small changes from directly downwind, hugely affect the situation.
In particular, even if the craft itself had wheels running in a direct line, it could have a cylindrical arrangement for its "sails", rotating vertically, so that in effect its
sails would be jibing even if the craft itself was going straight.
Again, an equilibrium condition examination (where the craft is traveling exactly at wind speed with any such arrangement not rotating) is not useful, because the equilibrium state is a
point that only exists in carefully controlled situations, and not in practice.
Plus, pressure differentials in the wind make the fluid dynamics even weirder.
As to the equilibrium scenario discussed ad nauseatum above, I have no opinion, because it is quite uninteresting to me, being overly simplified and unrealistic.