I understand that a bitcoin node will broadcast a new mined block to all the other nodes as quickly as possible so that the other miner nodes can work on the longest chain. But why would a node help other nodes by broadcasting to them the longest chain?
What if most of the nodes don't broadcast the longest chains in order to delay the mining of other nodes?
But why would a node help other nodes
Because anyone with any stake in Bitcoin needs the existence of a healthy Bitcoin network in order to be able to use their money and for it to retain any value.
Since the network is a mesh, there are, in general, no strategic nodes that can maliciously act as a bottleneck or filter. I would expect miners to take care that their selected peers are diverse.
Even if most nodes were malicious, it is very likely that almost any honest node would be in contact with at least one other honest node and would detect the malicious nature of the malicious nodes.
