One has to weigh the cost of preventing something from happening vs. the benefit of preventing it.
Humans will obviously become extinct on Earth at some point. We are a young species and we adapt slowly. It takes us over 10 years to reach sexual maturity, we have few births per woman/year, each member of the species requires substantial resources which severely limits the number of humans that can be on the Earth at any one point in time. This all leaves comparatively few opportunities for genetic alterations as opposed to, say, cockroaches. To make matters worse (although, as civilization declines, this will no longer happen so it's a temporary impediment), we interfere with natural selection via medical procedures and social programs so resources are consumed on "survival of the weakest" rather than on "promoting the strongest".
So, it's only useful to consider the chances of a catastrophic asteroid strike before we become extinct via other mechanisms. An asteroid strike 100 million years from now is completely irrelevant to humans as there will be no humans to experience it (or to maintain the infrastructure to prevent it). More adaptable species will survive it anyway.
Not as relevant to this specific case, but to be considered in discussions about extinction of the human species in general. Extinction of the human species is not necessarily the worst thing that can happen to humans (esp. since it's going to happen anyway). If the cost of delaying human extinction by N years is so high that all humans live in substantially less "comfort" for the remaining M years of human existence and N << M, it's likely incurring the cost of delaying extinction makes no sense (esp. to someone whose lifespan is much, much less than M or N).
Its analogous to, hypothetically, offering a healthy 30 year old two options. The first option is eating a distasteful, but extremely healthy, calorie restricted diet which will leave them feeling weak all the time but they will live, on the average, to be 87 years old. The second option is to eat pretty much whatever they want to enjoy and maintain a caloric count that does not interfere with their daily life or motivation or pleasure but they will live, on the average, to be only 86.75 years. A rational person would, I think, choose to live in comfort for their remaining 56.75 years rather than to live another three months but at the cost of being in discomfort and too weak to do much for their remaining 57 years.