That DNS is decentralized does not really mean anything when you consider how easy it is to uncover where a site is hosted, and understand which points you need to hit to take a site down. Whilst we can do interesting things at the network level to mitigate (think Cloudflare, anycast, mirroring, etc), the services sitting behind DNS are still exposed like a sitting duck.
I mean if we really wanted to DDOS Cloudflare, we just exhaustively gather all the raw APEX/Naked IPs of their edge nodes then stress them, but I imagine Cloudflare doesn't advertise their list of IPs and they're closely guarded, so attackers are left in the dark. But such an attack is plausible.
What we do need are antifragile protocols like BitTorrent/IPFS/Bitcoin which infact reward swarm behavior, instead of punish it.