But why would a new entrant get the chain from a node that already exited, and why would this affect anyone other than the entrant itself? I assume the new entrant is itself liable if it copies the wrong chain, because other nodes will vote against it once it starts operating, so it will make an effort to get the correct chain (maybe by buying it from current nodes and ensuring they all provide the best chain). So maybe you would have some kind of cartel of running nodes that may or may not allow new nodes to enter, but I don't see a critical network-destroyig issue here.