Let's take the example of climate change.

One of the lasts reports on climate change explained that we can't stop climate change while still using our capitalist approach, that revenue needs to drop and change our paradigm completely.

This is a realistic approach, one that reviews the data and explains what needs to be done in order to fix things. But, the persons that needs to do something are the ones taking advantage of the current system, and sell the optimistic position that we can fix things while keeping the same system, again, something that was proven completely incompatible.

So, the two realistic positions are:

a) people in power will not change in time, and humanity as we know is doomed.

b) we need to make radical changes and quick, making a lot of sacrifices in the process, in order to have a possibility of surviving.

None of this positions are optimistic, and both can be defined as pessimistic in different ways. Optimism never helped to change things, it only helps to keep the status quo, if the status quo is good, great, that is a good thing, but most of the times is not, so optimism in general is negative.

Of course, going to the extreme in pessimism is also not good, because extremes are bad, but the spectrum of options in pessimism are much more positive than the ones in optimism.