[zrm]

Modern P2P networks already prefer closer peers. This also happens naturally because closer peers generally have fewer bottlenecks between them, so if you connect to many random peers, the ones that are e.g. on your LAN will intrinsically end up doing most of the sending.

This is often more efficient than a CDN because a CDN won't have a box on your LAN even if there are multiple peers there.

And a CDN is only efficient to begin with if there are already multiple peers on the same ISP's network. If you're the only person on the ISP's network who wants some particular content then it has to come over a transit link just for you anyway. If you're not, you can get it from the other close peer(s) in a P2P network and not use the transit link.