And mankind's successor will match the same criteria. But there is no guarantee that it will be descended from us, and it will suck if we happen to be the generation that is going to win that lottery ticket.
Yes, the risk is low. We do things to mitigate low-risk things all the time. I also don't think this is going to happen in the next million years (statistically a one in sixty chance). But it behooves us to be able to more accurately gauge that risk. Right now, we can only give a very granular risk assessment. Funding the tracking of large asteroids is worthwhile on a number of levels, the scientific value being one of them. Being able to track rogue comets would be useful, too, but is much harder (and also a lower risk). If we could track these objects, which costs a fair bit of money, we would have a better window to deal with such an event. Like Hawking said, having people on different planets would also mitigate that risk.
I'm confident that life on earth will continue for the next 2 billion years, but I'm selfish enough to want that life to include descendents of humans.