I think you are looking at it from a very narrow perspective. Yes, it may not be feasible to do 5 million polygon texturing in substance painter, but who says it have to be 5 mil - Nanite is just a tool in the toolbox to make your life a lot easier (from the look of it). There are still a lot of unknowns about the limitations, but this will enable, not just games but other media too, to tap into the unreal engine workflow that wasn’t possible before. I’ve worked with offline-rendering professionally for 9 years and used ue4 for 3 of those years on private projects, and I’ve been pushing for a shift over to ue4 since rtx became a thing, and Nanite and Lumen is going to help make that transition a lot easier.
Virtual production is also going to benefit from this. You can pretty easily run out of VRAM in ue4 if your not careful. Even at a locked frame rate at filmic 24 FPS and a lot of overhead - you have to plan your scene as it currently is. It ain’t just a question about how many polys you can push on a single model.