This piqued my interest (and skepticism) immediately because, while I've never heard of optimistic bias before, I have heard of pessimistic bias. The latter is well known and well studied. We can't have both an optimistic bias and a pessimistic bias, can we?
Well, in principle, we can. The authors talk mostly about our beliefs regarding our own future as being overly optimistic. Pessimistic bias, on the other hand, relates to peoples beliefs regarding others, e.g., their estimation for the future of the country as a whole. It is of course possible that we could have both of these biases at once.
In fact, there are other biases that act this way. Consider, for example, the fundamental attribution error. People attribute more of their own behaviors to circumstances than they should and they attribute more of other people's behaviors to their innate characteristics (e.g. personality) than they should.
So I certainly can't rule out that this is the case.
However, the authors do not present much compelling evidence. They give only two examples with actual statistics (i.e. two examples that are not just anecdotes). Here is one:
A survey conducted in 2007 found that while 70% thought families in general were less successful than in their parents' day, 76% of respondents were optimistic about the future of their own family.
Unfortunately, this is not evidence that either one of these beliefs are false. All we know is that they cannot both be true. They might be perfectly right about their own future.
Funnily enough, this example comes up in the literature on pessimistic bias. We actually know for a fact that these people are very wrong in their first belief: the vast majority of all families are better off than their parents. Pick whatever statistic you like and the numbers will most likely look better today than they did back then.
What about the second belief? I think all the evidence suggests that this estimate is if anything too pessimistic. If 24% of children were worse off than their parents, that would significantly buck the historical trend.
I'll keep my mind open regarding optimistic bias, but at this point, I see no reason to believe it exists.
