I've seen a list of hardcoded seed nodes in the Monero code. If somebody DDoSed these IPs until their servers crashed, would it be enough to bring down the Monero network? If not, what effect would it have?
It would not bring the monero network down. At most, it would prevent new nodes from adding to the network.
The seed nodes are "well known" nodes which act as the first port of call for newly setup nodes. Once a node is already in the network, it does not need the seed nodes. If a node stops and is then restarted after a delay, the list of known peers is kept in the
p2pstate.bin file, so seed nodes are not needed either (though they are still contacted).
If a DDoS happens, new nodes still have the ability to connect to the Monero network from scratch with the
--add-peer argument to
./monerod --add-peer 18.104.22.168:18080