Conservation of momentum is a physical law, that is, we're not making that up, we always see it. The Navier-Stokes equation is a continuum mechanics model of fluids with interesting mathematics, but it's not fundamental, it relies on conservation of momentum and additional reasonable assumptions about the fluid, in the same way that for instance the van der Waals equation is reasonable but it's not a law, you find deviations and there are better models.

As far as we can tell the laws of thermodynamics are laws indeed, that's why no one is proposing perpetual motion machines of the second kind any more.