Comment by cortesoft

As someone who has worked for a CDN for over a decade, this is what most big customers do. Under normal circumstances, they send portions of traffic to different CDNs, usually based on cost (and or performance in various regions). When an issue happens, they will pull traffic from the problem CDN.

Of course, if a big incident happens for a big CDN, there might not be enough latent capacity in the other CDNs to take all the traffic. CDNs are a cutthroat business, with small margins, so there usually isn’t a TON of unused capacity laying around.