I can see where you're coming from, but I think I'd prefer to describe practically all stable software as living in an unstable equilibrium in the usable region of state-space. When the stabilizing force of security patches, certificate updates, updates to new hardware requirements, and so on and so forth disappears the software falls out of the usable region of space into the, I suppose stable equilibrium, of unusable software. And this fall happens quite rapidly in the case of a linux distribution.

Applying the word "stable" to things in the unusable region of state space seems technically, but only technically, correct.