Wiping out 90% or so of the human race without killing everyone seems unlikely in general.

Less likely than killing 100% of the human race? Why?

Remember that humanity went through bottlenecks where the total population was reduced to tens of thousands scattered in pockets of hundreds to thousands. Humanity survived the Toba super eruption in prehistoric times, and would probably survive the Chicxulub impact if it happened today.

Other than an impact powerful enough to sterilize the biosphere, I don't see many things capable of obliterating the human species in the foreseable future. Pandemics don't have a 100% kill rate (at least the natural ones, maybe an engineered one could, but who would be foolish enough to create such a thing?)