long answer: 

Complexity: A Guided Tour - Book by Melanie Mitchell

short answer - look up scale free networks

micro answer - yes, one of the properties of decentralization is how it reacts to failure. 

Nostr is not a transport layer, for example I'm working on using Nostr over Reticulum/ LoRA which is a different transport layer yet using the same Nostr protocol. I believe a good question about Nostr decentralization is not about decentralization of TCP/IP, even though it inherits its properties if you use it through this protocol. 

The good question is how resilient the network is against attacks on relays, but you always need to have a dynamic look at things - what will the network do after the attack is as good of a question as what happens immediately after a successful attack.