This is a "good" model from a conceptual standpoint. It's the model Isaac Newton himself used when he tried to model the concept of aerodynamic lift. However, modeling air as a series of little balls bouncing off one side of the sail ignores the contribution of the air on the other side of the sail.
Air consists of a bunch of "little balls", not just a few, and they don't just collide with the sail, they collide with each other. A lot, and very frequently. The missing piece in your model is that air is deflected on both sides of the sail, not just one. And the contribution from the 'front" side is actually greater than the contribution from the side you show.
If you follow this analysis of this model to the end (as Newton did) you'll find that the lift is proportional to the square of the sine of the angle of attack. This is incorrect. A correct analysis that incorporates deflection on both sides of the sail says that the lift is proportional to the sine of the angle of attack, not the square of the sin. Since sin(x) <= 1, sin^2(x) <= sin(x). This analysis grossly underestimates the amount of lift, especially at the small angles of attack employed by airplanes.
A historical note: some folks accepted Newton's analysis at face value and concluded that airplanes were impossible because you can't get enough lift. Fortunately, this wasn't everyone.
Note 2: Yes, Newton got this particular analysis wrong, but Newton's laws of motion still apply. Fluid dynamics was in its infancy when Newton tackled the problem, and nobody's perfect.
Note 3: When people talk about the "Newtonian explanation of lift" they are not talking about this flawed analysis.
BTW, NASA calls this model "Incorrect theory of lift #2
ADDENDUM: Forgot to mention that for hypersoinc flight at low atmospheric pressure this model is applicable. For instance, it describes the space shuttle during re-entry quite well. If the air molecules are few enough and the airfoil is moving fast enough we can neglect molecule-to-molecule interactions and concentrate on the molecule-to-wing interactions. But for sailboats (which don't come close to mach 1) at reasonable atmospheric pressure, the 'bouncing balls' or 'skipping stone' model falls short.