1. People like you thought airplane or moon landing are unrealistic and unfeasible, but they're wrong
2. Just because there's no technology to do it now, doesn't mean there won't be technology to do it in the foreseeable future. This is what PR is doing, developing the technology.
3. And who says the developed technology won't be used on earth too, it can benefit both earth based mining and asteroid mining. The material from asteroid is not meant for Earth anyway.
4. The extraordinary claim is not we'll be mining asteroid, it's the claim that "It's over, and we're going nowhere"
5. Space may be dead for you, but the kickstarter campaign proves it lives on in many people's hearts, so go ahead and drown in self-pity, we got asteroids to mine.