I have 12 years of professional experience. In that time, most of what I worked on shipped. It would be a serious failure if we committed to a feature and were unable to deliver it. To be fair, I have tended to work on mature products, which have predictable lifecycle. Perhaps you've had a different experience because you've worked for startups or on early development?

Anyways, the way I see it, if I'm OK with my project being cancelled, I am working on the wrong project. We should not be comfortable with delivering results "once in a while."