How is motivation influenced by chance of reward?
Our subjective estimation of probability is affected by many irrational factors, one of which is the accessibility of exemplars (Availability Heuristic). Since we usually hear about lottery winners, and not so much about those who didn't win, we over-estimate the probability of winning. It may be similar with your estimation of the probability of the project becoming irrelevant - it is affected by how easy it is for you to imagine the project failing, something that can be affected by lots of irrational factors (like how self-confident the project leader is, or even worse, how much you like him/her).

Dan Gilbert has a nice TED talk mentioning the Availability Heuristic in lottery and other similar biases in estimating gains and probabilities.