That's a really good point. I'm inclined to think that there's an asymmetry that tips the balance towards suffering, like maybe the fact that they only exist for fractions of a second is distressing, or maybe pleasure requires a more fine tuned structure. But it's hard to avoid anthropomorphizing the randomly generated brains, so my intuitions might not be correct. There's also the whole negative utilitarian vs total utilitarian debate. 

Great post! I think cosmic threats are definitely worth learning more about.

Only one thing bugged me. When you say the heat death of the universe is "the 3rd law of thermodynamics", it confused me because there's already a 3rd law of thermodynamics and, as far as I know, it has little to do with heat death.