At least for Google's case YouTube videos are usually transcoded in idle datacenters (for example locations where the locals are sleeping). This means that the

At least for Google's case YouTube videos are usually transcoded in idle datacenters (for example locations where the locals are sleeping). This means that the cost of CPU is much lower than a naive estimate. These new accelerators can only be used for transcoding video, the rest of the time they will sit idle (or you will keep them loaded but the regular servers will be idle). This means that the economics are necessarily an obvious win. Of course if you do enough transcoding that you are buying servers for the job then these start to save money. So I guess someone finally decided that the R&D would likely pay off due to the current combination of cyclical traffic, adjustable load and the cost savings of the accelerator.