I doubt there is much efficiency gain when most distro's compress the packages and ISO's anyway, the few bits you could spare would most likely fall towards the installer ISOs, which are always mostly the same. But since nobody cares about the old ones, there isn't much win other than getting a tiny bit more bandwidth from people who don't offer the newest version yet.
There would also have to be a very high uptime guarantee, which means either every user of the distro operates a IPFS node by default, which nobody wants because IPFS is extremely chatty and there is such a thing as "servers in private subnets with limited internet access" that people don't want to send out arbitrary data to the internet. The next option would be opt-in, which means nobody will do it because everyone is lazy. Last option; simply maintain status quo.
There would also have to be a very high uptime guarantee, which means either every user of the distro operates a IPFS node by default, which nobody wants because IPFS is extremely chatty and there is such a thing as "servers in private subnets with limited internet access" that people don't want to send out arbitrary data to the internet. The next option would be opt-in, which means nobody will do it because everyone is lazy. Last option; simply maintain status quo.