It's actually better if 1000 different people each run a 40Mbps exit node than if one ISP runs a single 40Gbps one. You don't want to centralize control over the exit nodes because it increases the chance that party could control every node in a circuit.
If you know which nodes they control you can easily avoid using them in the same circuit. But how are you supposed to know that? There is a configuration option to list other nodes you operate for exactly this purpose, but someone staging an attack is obviously not going to use it.
