What if the TDTs that you're playing against, decide to defect unconditionally if you submit a CDT player, in order to give you an incentive to submit a TDT player?

That's a good point, but what if the process that gives birth to CDT doesn't listen to the incentives you give it? For example, it could be evolution or random chance.

Here's an example, similar to Wei's example above. Imagine two parallel universes, both containing large populations of TDT agents. In both universes, a child is born, looking exactly like everyone else. The child in universe A is a TDT agent named Alice. The child in universe B is named Bob and has a random mutation that makes him use CDT. Both children go on to play many blind PDs with their neighbors. It looks like Bob's life will be much happier than Alice's, right?

We can't all act on unusual reasons and end up doing the same thing, after all.

What force will push against evolution and keep the number of Bobs small?