My point is about (1) the broader ecosystem of tools that may need to interoperate and have easy access to "SHA256 the whole file" and (2) the fact that not everything is a tar file that the Nix tools can process. So yes, that's the "only" case.
So what about the IPFS CAR format? https://car.ipfs.io/, it would fulfill a lot of what I expect from NAR too. NAR or CAR, I don't care, I believe the content is what matters, not the container format.
If I have a box with an apple in it, I don't care about the box, I care about the apple inside. If it's not an apple, I don't want to eat it.