For each individual video, it is a onetime CPU cost, but not a onetime bandwidth cost.

For ongoing operation of the platform, at scale, you might argue that it's an ongoing CPU cost as more and more videos are added constantly, but the bandwidth to distribute those videos goes up at an equal rate on average.

I don't know any actual numbers, but it seems very plausible to me that a vast increase in CPU cost for a minor decrease of bandwidth cost could still be a welcome trade-off.

If you're making an argument that availability of CPU power at that scale is a concern, it seems like YouTube has the option of Google's cloud services as an excellent candidate for massive, interruptible compute power.