Well, technically, nothing prevents every publisher from running their own hub (we host some at Superfeedr). Also, we even encourage publishers and subscribers

Well, technically, nothing prevents every publisher from running their own hub (we host some at Superfeedr). Also, we even encourage publishers and subscribers to both publish to several hubs and subscribe to several hubs as well, so that they can avoid any bottleneck or reliance to a single hub. The only 'burden' on their side is to deal with de-duplication, but that's quite easy.