I'll give it ago, probably revealing my ignorance.

The spam-debate is said to be related to PoW. But, PoW has one crucial role, not related to spam (from my perspective). It simply coordinates the longest chain--nothing more. Of course blocks need to be in consensus to be included; so the next mined block is held to consensus by the node runners. A miner-node could submit their "proof of work" on a non-consensus block, but it will not be included.

Next, other protocols are said to have a centralizing effect because of the use of "social consensus". In the case of email, the protocol is decentralized; there are decentralized approaches to dealing with spam, I know people who run their own server without problems. Users choosing a walled-garden such as gmail, and gmail being hostile seems to be one of the bigger problems. I'm not sure where the centralizing effect comes from, likely users choosing walled gardens for connivence (?). If bitcoin had a walled-garden, maybe we could have a spam-free chain! But I am not sure it was the fault of "social consensus". If we had a "social consensus" to reject centralizing forces, we may had better systems/protocols.

I guess the argument is that fees are the spam mitigation (as compared to email which does not have fees). There have mostly always been(?) been >0 sat/vB fees. We have mostly always had spam.

The question is, how do we truly reject any centralizing force? There is a complex set of motivations and incentives. Fiat addiction breeds centralization.