FFmpeg's costly functions are all written in hand optimised assembly

If by 'String/Quantum' you mean String Theory and quantum physics then you are wrong on the latter (and somewhat even the former). Quantum physics doesn't replace classical physics nor are they necessarily in opposition, and quantum physics is as much a theory as classical physics; so bashing one for being 'theory' is just as true for the other. And quantum physics is certainly in common use as you simply cant do anything at the atomic level without it. For example, any modern computer would not be able to function if quantum physics wasn't used to inform their design; in the same vein a modern computer would not function if classical physics was used to design them. It's important to remember that the word 'theory' in this context doesn't mean unproven, rather it describes a collection of confirmed, falsifiable, explanations of the natural world.

As for String Theory, it shouldn't be thought of as equivalent in scale to quantum physics, it's really just an optional framework within quantum physics that attempts to describe the fundamental nature of particles in a way that supports quantum gravity. Due to this its usage is confined to theoretical physics and is dependent on which aspects of a system is being investigated, but it's still used in some situations as its one of the best supported tools available.

I guess my main point is that quantum physics isn't fringe theory that shows up only in theoretical work, it's very much a requirement for all fields and is thereby prevalent and very much in common use. I have a CS degree and many of my courses touched on quantum mechanics, from pnp/npn transistor design to quantum-annealing/gate proof cryptography, without getting too into the mechanics/math as we were not physicists.