The fact that AIs will be able to coordinate well with each other, and thereby choose to "merge" into a single agent

My response: I agree AIs will be able to coordinate with each other, but "ability to coordinate" seems like a continuous variable that we will apply pressure to incrementally, not something that we should expect to be roughly infinite right at the start. Current AIs are not able to "merge" with each other.

Ability to coordinate being continuous doesn't preclude sufficiently advanced AIs acting like a single agent. Why would it need to be infinite right at the start? 

And of course current AIs being bad at coordination is true, but this doesn't mean that future AIs won't be.