Comment by zamadatix

(Having also worked on DDoS mitigation services) That "entire /64" is already hell of a lot more granular than a single CG-NAT range serving everyone on an ISP though. Most often in these types of attacks it's a single subnet of a single home connection. You'll need to block more total prefixes, sure, but only because you actually know you're only blocking actively attacking source subnets, not entire ISPs. You'll probably still want something signature based for the detection of what to blackhole though, but it does scale farther in a combo on the same amount of DDoS mitigation hardware.