The analogy of the resting stone isn't entirely incorrect. It is still doing something and it can be argued that all the forces acting on it are in equilibrium (good word huh!
). That will continue to be the case until it is affected by an acceleration - thrust created by someone kicking the stone, perhaps?
Anyway, it doesn't adequately illustrate the point but the reality is that in S+L flight, all the forces acting on an aeroplane are similarly in equilibrium until something is changed. I've deliberately avoided any reference to 3 or 4 force (or whatever) models because, even if there are a thousand forces acting on the aeroplane in S+L flight, they are
ALL in equilibrium.