The problem you see with AV1 streaming isn't the film grain synthesis; it's the bitrate. Netflix is using film grain synthesis to save bandwidth (e.g. 2-5mbps for 1080p, ~20mbps for 4k), 4k bluray is closer to 100mbps.
If the AV1+FGS is given anywhere close to comparable bitrate to other codecs (especially if it's encoding from a non-compressed source like a high res film scan), it will absolutely demolish a codec that doesn't have FGS on both bitrate and detail. The tech is just getting a bad rap because Netflix is aiming for minimal cost to deliver good enough rather than maximal quality.