If you DOS one client , the clients it's connected to will see the connection timeout and simply drop it . This drops them below their set number of connections and they wil look for new connections with other clients .
Thus that one unfortunate client that got DOSed gets kicked of the network but the network will recover to optimal condition in no time .

It's indeed easy to DOS one client but not the network .
You can't compare a DOS attack against a single computer or server with an attack against an complex network like Gnutella .