If a 100 Gbps link goes down, and all the other links that could carry that route are only 10 Gbps... it goes downhill quickly.
Too much traffic is concentrated at the major carriers which carry a large amount of traffic over relatively little fiber (cause its cheaper) and when that fiber disappears we lose a massive amount of capacity...
Just to add to that, sometimes the redundancy doesn't work out as planned, maybe another weird set of circumstances, or some routers can't handle spikes in load as the network reconverges, causing other reconverges, confusing bgp routing peers, etc. Fun stuff!
