If the decentralized network allows for some kind of targeted broadcasting, it becomes attractive for spamming (e.g: email).
If the decentralized network allows for concentration of responses on something, it becomes a potential tool for a DDoS attack (e.g.: DNS amplification).
So running a node should be somehow expensive, but the expense should be written off if the receivers of the message endorse it, by a one-time action, or automatically by subscribing. An initial credit would allow to establish an audience.
It looks like a perfect use case for a cryprocurrency of sorts %) But this means expensive coin generation, and distribution of the huge ledger across all nodes. That could be delegated to some specialized nodes, but here comes centralization again!