Yes, you can fetch directly from SWH, but then it isn’t clear how to do so as a substitutor without more changes to nix. Conversely, you can use the IPFS gateway without having IPFS installed — that has improved since we did the Nix IFPS work so we could add support for that.
SWH is supposed to be an archive of last resort to potentially tons of people flooding their API with similar requests just waste’s the non-profit’s bandwidth. IPFS however can act as a CDN.
With the spread of Radicle IPFS is no longer the only content addressed data source and we should maybe make the debate more broad to encapsulate both forms of content addressing
If only there were a data model that was more general and could represent both. 
https://ipld.io/
Thats what I am also looking into, and thats why I am trying to get go-car: init at 2.16.0 by malikwirin · Pull Request #423507 · NixOS/nixpkgs · GitHub merged before thinking about ways to implement nix functions to convert these into each other