One argument for a large number of humans dying by default (or otherwise being very unhappy with the situation) is that running the singularity as fast as possible causes extremely life threatening environmental changes. Most notably, it's plausible that you literally boil the oceans due to extreme amounts of waste heat from industry (e.g. with energy from fusion).
My guess is that this probably doesn't happen due to coordination, but in a world where AIs still have indexical preferences or there is otherwise heavy competition, this seems much more likely. (I'm relatively optimistic about "world peace prior to ocean boiling industry".)
(Of course, AIs could in principle e.g. sell cryonics services or bunkers, but I expect that many people would be unhappy about the situation.)