Film grain is so expensive to encode that in bit-rate-constrained situations you would be forced to choose between obvious ugly compression artefacts, a reasona

Film grain is so expensive to encode that in bit-rate-constrained situations you would be forced to choose between obvious ugly compression artefacts, a reasonably good image without grain, or a reasonably good image overlaid with inauthentic grain. Besides, "authentic" is such an overhyped pile of poo anyway. Everything is a reproduction; what matters should be that the artistic intent remains unaltered.