If namecheap chose to stop assigning all customers the same list of DNS servers, it would benefit namecheap as well. There's 200+ people in the queue for live chat right now :)
It's not DDOS mitigation or H/A, or some other high end feature.
The only way this would be successful is if the "customer IP's" were spread accross separate networks, and the announcement for the attacked network was sent somewhere else.
Assigning customers across lots of IP's in the same /24 isn't going to do anything. A volumetric attack is still going to succeed there.
In this landscape of ever-dwindling portable IPv4 subnets, it's harder and harder to get a /24. You won't get an assignment if your justification is "to fight a DDoS."
It's not DDOS mitigation or H/A, or some other high end feature.