I think you're missing the point of these appliances. Netflix has plenty of capacity to host all its content in central locations.
These appliances are installed at the ISP offices so that the content is as close to the subscriber as possible. That way the quality of the video is not dependent on the quality of the long-haul network from the ISP back to Chicago, Dallas, Ashburn, London, Frankfurt or wherever.
It also reduces the IP transit costs of the ISP, which they are typically paying for based on utilization if they are not a Tier 1 like AT&T. It also reduces their transport costs. If the ISP has to upgrade from two 10gbe pipes to four, that's probably going to hit their bottom line for $10k to $20k a month. If installing some caching appliances helps them delay that upgrade by a year, that's a massive benefit.