I think what you're saying is that a successful movie must be enthralling, and what makes a movie enthralling is fairly universal (protagonist with a clear goal....).

But genres fulfill (or frustrate) different expectations (Nobody dies at the end of a romcom, for example, though somebody may die in the middle--Four Weddings and a Funeral). Rules of the Game is essentially a romcom until someone dies at the end. Then it becomes something else.

So while all genres adhere to the same rules (invariants), they play to different expectations (variables).