How do decentralised systems deal with all nodes being killed

In relation to DHT’s (distributed hash tables), how do decentralised systems deal with all nodes disconnecting and thus having its routing meta data invalidated? i.e with the implementation of DHT’s in torrent clients, what would happen if all nodes (including the root/parent) were to become inactive? Would it mean all subsequent nodes that join wouldn’t be able to see anyone thus invalidating the system?

If this is the case, then is it wise to only use decentralisation on a system that will have constant connections? Or is it the case that its recommended to have the root/parent node be a centralised sever with the dedicated task of keeping the system alive, to create a hybrid mix of centralised & decentralisation?