Conventional wisdom is wrong. Buy into it at your own peril because, if you don't watch out people will manipulate and bully you with the supposed but fake virtue of optimism.
It's not that it's better to be pessimistic but that optimism and pessimism are two sides of the same coin. To be optimistic about one alternative is to be relatively pessimistic about other alternatives. "This plan will work," means at minimum, "We don't need another plan" but more likely means "The other plans are less likely to succeed."
Optimism and pessimism are as inseparable as inhaling and exhaling. They're "reflexively antagonistic" the way to tighten your bicep you must loosen your triceps, and visa versa. In debate you're opponent who is optimistic about his plan is, by definition pessimistic about yours. Don't let him claim the high moral ground by saying that since he likes his plan, he's an optimist.
Don't buy the currently popular malarkey about the power of positive thinking. Being positive about one thing is being negative about alternatives by comparison. Positivity isn't a virtue it's a focalizer, a way of saying "I'm prefer this; not that." You can't prefer everything any more than you can inhale everything. We have finite focus. To focus here means not to focus there. Positivity is a necessity, but then so is inhaling. That doesn't mean you should or can inhale all day. The question is not whether to be positive but what to be positive about.
When optimistic bullies say, "Well, at least I'm being optimistic." They add insult to injury. Not only are they optimistic about their preferred plan (in comparison to yours which relatively speaking, they're pessimistic about), they also discredit your planning skills by calling you "pessimistic."
Since optimism and pessimism are two sides of the same coin, there are ways to bully through pessimism too. Since on the optimism/pessimism spectrum, optimism has the virtue-monopoly, pessimistic bullies can't retaliate by calling their opponents "optimistic." But that doesn't mean pessimists are without recourse. They can add insult to injury too by evoking the various sins associated with optimism, accusing you of being over-optimistic, unrealistic, engaged in magical thinking, and living in Lala land."
We treat "optimist" and "pessimist" as though they were descriptive labels applied neutrally, like calling that tall thing with the leaves a "tree." They're not descriptive, they're opinions in disguise, opinions about who's right and wrong, made unfairly strong by the moral weight the terms "optimist" and "pessimist" carry.
Optimism and pessimism are relative concepts in two senses. One sense is that they're merely subjective assessments relative to someone else's. If I think a plan has a better chance of success than you do, I'm an optimist but only relative to you. Someone else might think the plan has vastly better chances of success than I do which would make me a pessimist, relatively speaking.
Optimism and pessimism are relative concepts in another way also. If I think a plan has a great chance of succeeding, I'm implying "relative to other plans."
So who's right about the plans, the optimist or the pessimist? Those terms tell you nothing about which plan will actually succeed.
There's a simple way to neutralize the power of optimistic and pessimistic bullying: Don't ever accept the use of those terms without them being followed by "...about X."
The statements "I'm optimistic" or "I'm an optimist" don't make sense really. They're open-ended, like saying "I throw..." They should prompt you to ask a follow-up question. What do you throw? What are you optimistic about?
"I'm optimistic about this plan" makes sense. It translates simply as "I'm betting this plan will work relative to the alternatives."