It almost sounds like Posner’s jumping on the personality-and-economics bandwagon:

Optimism is also a personality trait, and, as it happens, one essential
to human progress. As I have argued elsewhere with reference to our
current economic situation, what Keynes called “animal spirits” and,
alternatively, confidence or optimism are essential to entrepreneurship
because of the great uncertainty of a business environment. Someone who
invests in building a factory that will not produce anything for years
is taking a big risk of failure, and because it is a risk that cannot
be reliably quantified he is taking a leap of faith, and he will not do
that unless he happens to have an optimistic outlook. It is not that
rationality implies such an outlook, but that rationality is not
inconsistent with it. Optimists are often disappointed, but sometimes
are richly rewarded for the risks they take; and as long as the
prospect for such rewards confers on them greater ex ante utility than
more cautious, pessimistic decisions would do, they are not behaving
irrationally. “Nothing ventured, nothing gained” is the credo of the
optimist and the terror of the pessimist, but neither reaction is
irrational. The optimist and the pessimist just have different

I like where he’s going, but I think he’s going a little too far.  Optimism could be unusually mild risk-aversion.  It could just as easily be a tendency to think the world is better than it really is.  Or as I’ve argued before, optimism could be a strategy and an attitude:

[O]ptimism, as I practice it anyway, is an attitude and a strategy,
not a description of the world. As an optimist, I try not to dwell on
boring careerists and derivative claptrap. Instead, I seek out the
exceptions to the rule and appreciate what I find. Just because the
average is low doesn’t mean that you can’t personally consume high
quality. And even when the quality I consume is far from ideal, I try
to mentally change the subject to another dimension where I have
blessings to count.

Regardless of which definition of optimism you use, it’s hard to see why optimism is “essential” for human progress.  Optimism probably gives progress a boost, but it’s highly unlikely that progress would grind to a halt if everyone had rational expectations and risk-aversion.