Set up correct secondary DNS servers.
If the secondaries had not been hosted at the same company, but instead at various companies around the world, the attack would have had no effect on anything but traffic.
This is, by the way, how multiply connected networks are supposed to work.
This could be easily accomplished at no additional cost by having a peering-pool arrangement between all the host registrars, so that we ended up with a multiply connected redundant network.
Kind of how we designed the thing to work in the 1960's and 1970's, and DNS itself in the 1980's.
But a lot harder for law enforcement to issue DNS-based takedowns on, of course. Since it would route around the damage and keep functioning. As designed.