Incremental build farms

Build farms are expensive to setup and maintain. Or are they? Amazon gives away free instance-years, and services such as Travis and Appveyor build open-source projects for free. Can these services be used to bootstrap a Linux distribution?

The Nix answer to these questions is to use a binary cache - a server in the middle that stores cryptographically-signed binaries. It can start as simple as a single node, with a Nix store, then expand to use Amazon S3 or another large-scale filesystem.
But why start there? We could instead start with a distributed filesystem, like Bittorrent's DHT or IPFS. Then sharing files is as easy as sharing the root location hash, and all we need is a simple GitHub Pages site that gives out the root hash for everyone to synchronize on.

The issue of trust still remains though. Who builds what? How does the hash get calculated? Is there a cryptocoin (filecoin)?

Another approach starts from a single moderately-powerful system; Gentoo is a source-based Linux distribution that can usually be compiled and installed overnight. Speeding up Gentoo (re)compilations is a worthy goal. But then how do we go from one system to a network of systems sharing binaries? Gentoo (portage) actually supports binary packages, but in practice this seems like it is never used publicly; instead it has a centralized system of mirrors for distfiles. This is probably due to culture and limitations in compatibility of binary packages; it seems like Gentoo is usually set up to compile packages for the specific CPU it's running on, as a side-effect of too many USE flags ("Gentoo is for ricers"). But doing some sane defaults like Arch shouldn't be too hard, and proper dependency management should alleviate all the compatibility / configurability concerns.