[ibz]

Very good point, I didn't think about the propagation function of the nodes. Indeed, a node would only propagate blocks it accepts as valid.

But how does that help the network except by saving some bandwidth?

In the end, "bad" blocks will and should propagate and should be rejected by every node that decides that the block is "bad". There isn't one node that is a gatekeeper for "bad" blocks.

I think that whenever people talk about consensus, they think about it in terms of how a democracy works - and that is wrong, because Bitcoin is not democratic. It's not like if you nave n nodes, you need n/2+1 to agree on something and all the rest will be forced to agree to their decision. It's up to every individual node to enforce rules for their own transactions.

[once_inc]

Nodes automatically forward valid blocks to all connected peers. This means that the initial miner would send it to, say, 100 peers, all of whom would reject it. Why send that block to anyone else, wasting resources and posing an attack vector (spamming the network with bad blocks, hogging resources and slowing/breaking the p2p network layer).