Will mentioned a couple of other possible ways in which UFAI fails to kill off humanity, besides distant timeless trade. [...] Although perhaps "hidden AGIs" is unlikely and you consider "potential simulators" to be covered under "distant timeless trade".

This is considered unlikely 'round these parts, but one should also consider God, Who is alleged by some to be omnipotent and Who might prefer to keep humans around. Insofar as such a God is metaphysically necessary this is mechanistically but not phenomenologically distinct from plain "hidden AGI".