Two flat regular mirrors do not form a cavity. Diffraction will always cause enough light to escape to make the setup useless.
In order for two flat mirrors to form a cavity they must be of infinite size so that light cannot escape. And the light in the cavity must be a plane wave (zero divergence).
Addendum after edit
Aha. You have one or two misunderstandings. Note that the orange lines are not straight. They are curved near the flat mirror. So what does that mean? What do the orange mirrors represent? The orange lines are perpendicular to the phase front. At the flat mirror the phase front is planar, and the orange lines are perpendicular to the plane of the mirror. If that mirror were partially silvered, the light that gets through would diverge in exactly the same way that it converges inside the cavity.
The field distribution in the cavity is not so simple. It is what's called a Gaussian beam. The profile of a Gaussian beam is displayed in this S.E. post. To make a cavity with a plane mirror the mirror must live where the phase front is planar, the narrow part of the beam, which is called the waist.
For a cavity to be made of two plane mirrors, the phase fronts must be planar everywhere, that is the wave must be a plane wave. A plane wave has zero divergence. It also has infinite cross-section, so the mirrors must be infinite.
Note that a Gaussian beam also has infinite cross-section. However, the field intensity drops off quickly away from the center so that very little light appears at the edge of the mirror. Very little is not zero, so there is always some loss from the cavity.