No asteroid large enough to kill billions of humans is likely to impact Earth in this century. After that, it's likely that humanity will be so advanced that it'll be able to either mitigate or deter an impact of any magnitude.
An overlooked possibility imho would be a catastrophic population decline[1]. Basically women not having children anymore. Japanese style, only worse.
It's difficult to imagine it leading to extinction, since there will always be groups, at the very least religious groups, who will keep having babies. But still, it wouldn't take much of a cultural change for it to expand to all women.
Not saying it's plausible, but to me in terms of population dynamics it's at least as much of a concern as climate change (by this I mean that I believe reproductive behavior will have more impact on demography than climate change).