Subpopulations of millions or tens of millions (0.1%-1% of total) are small, not large. From the viewpoint of species preservation, decline in some of them is insignificant; complete extinction of some such subpopulations would reduce diversity but still wouldn't threaten the species as such, only the affected subpopulations - and even then, due to widespread migration the diversity would be preserved in a larger extent than any of the somewhat threatened animal species.

An event that kills all humans in one continent would be horrible but would not present "a threat to humans" in the meaning of species survival/preservation.

An event that kills 90% of people worldwide would be horrible but again, would not present a threat to species survival, the humans were just as dominant in 1700s with less than a billion of us than we are now with 7.4 billion.

Global warming can really hurt hundreds of millions of people, but it doesn't have the potential to be an extinction event, you need far more serious threats for that.