I mean, if a firewall get destroyed it isn't then allowing the attackers into the internal network, if a DDOS attack actually fries the firewall rendering it inoperable, the worst that happens is no internal traffic is reachable from the outside internet and the internal network can't reach the outside internet, it's a plug in the flow of information, not a battering ram opening up unrestricted data transfer. A DDOS attack doesn't break through anything, it only jams it up.