There are other content addressed distributed file systems that are available too, think just because IPFS was first doesn’t mean its the best choice.
In fact the design choices of IPFS, if it gets popular , it’s going to have problems scaling to million of nodes, due to inefficiency in content discovery/publishing…it’s just one large DHT, but maybe they will copy the idea of topics soon.
However, let the best distributed content addressed file system win.
Many solutions you can’t directly share the /nix/store, but you have to have a copy dedicated to distributing over IPFS which leaves you quite a lot of redundant data. 
However, i need to check the IPFS nix extensions, and see if/how they address this.
I’ve reviewed the IPFS patches for nix, there certainly has been an extensive amount of work gone into them judging by the size of them.
Interesting stuff!