I can right now look at every single applicant to YC, predict they will fail, and be right the overwhelming majority of the time.
I can look at every amateur scientist who has found "a problem" with mainstream science, predict that they are a crank, and be right the overwhelming majority of the time.
I can look at every "breakthrough in battery technology," predict that it won't be on the market in 20 years, and be right the overwhelming majority of the time.
Nevertheless, there are YC applicants that found successful companies. Amateur scientists have contributed to scientific breakthroughs. Battery breakthroughs have made it from lab-to-market in much less than 20 years.
Depending on the subject, people's natural state is optimism even if they think they are being realistic. [1] and [2] have a bunch of interesting references on this matter
Appearing smart to dumb people isn't hard, so it doesn't count as a negative about pessimism :)
I won't make a judgement about it being usually right, but being pessimistic can be a tool to counteract the standard optimistic stance if you know people are usually optimistic (aka their expectations overshoot reality) on the subject
OK, one example was a bit flippant and easy to dismiss, but:
"No, this isn't going to bring us meaningfully closer to economically-viable fusion power"
"No, this isn't the War to End All Wars"
"No, you're not going to keep your New Years fitness resolution"
"No, your company's not going to succeed"
"This miracle chemical's probably gonna turn out to be bad for our health. I look forward to finding out how fucked I am because of it in 40 years when someone gets around to doing anything about the alarming study someone will likely do in 10 years only to be ignored. If I make it that long. But at least we can make things fire-resistant 6% cheaper than before!"
A lot of pessimism is of the "this shitty thing that no one capable of fixing has any incentive to fix will continue to be shitty" variety. That is usually right, even if it is more pessimistic than "I'm going to fix this shitty thing even if it kills me". This basic pessimism is still less pessimistic than "nothing is shitty anywhere I can't hear you nananana!"
