Yeah, when people start talking about how great positive thinking is, I always think, "Sure, as long as things are going well."
Your mindset is, to some degree, a prediction of the future. For that prediction to be helpful, it needs to be fairly accurate. Now many people will point out that predictions can be self-fulfilling, and a positive expectation is more likely to lead to a positive outcome. It's true. But it can also leave you unprepared for a negative outcome.
I think positivity needs to be measured, constrained, and tempered by a realistic assessment. It's useful to consider the what happens if things don't go well, and prepare for things to go very badly. Perhaps more importantly, I think it's important for us all to understand that different people just seem to have different mindsets, different approaches, and different personalities. Whatever your mindset, it can be helpful to have someone around who is very positive, because they can help to keep us all motivated and moving forward. But it's also very useful to have a pessimist around, who will point out the problems with your plans, force you to confront some uncomfortable truths, and rain on your parade a little bit.
People of all kinds can be useful and valuable, so it's important to not shit all over pessimists all the time. As if they don't have enough to worry about, without y'all treating them like they're useless.