Cudos to Andreas Giger for noticing what most of the commentators seemed to miss: "How can utility be maximised when there is no maximum utility? The answer of course is that it can't." This is incredibly close to stating that perfect rationality doesn't exist, but it wasn't explicitly stated, only implied.

I think the key is infinite vs finite universes. Any conceivable finite universe can be arranged in a finite number of states, one, or perhaps several of which, could be assigned maximum utility. You can't do this in universes involving infinity. So if you want perfect rationality, you need to reduce your infinite universe to just the stuff you care about. This is doable in some universes, but not in the ones you posit.

In our universe, we can shave off the infinity, since we presumably only care about our light cone.