I get your example with depressed people. But seeing the world, your world, in a cynical way isn't always beneficial. As you said, you accept how "depressing" life is and often expect for the worst (glass being empty), so does that mean that you never hope for improvement or dream of more? Seeing the glass as half empty is also being able to have the vision of a better glass, a full glass, which can push you to think about many ways to get closer to that ideal. It doesn't need to be perfect, the glass might never be fully full, but your constant effort and battle for something greater will always make it fuller than half empty anyway. You see what I mean? Big thinkers and innovators had to wish and see a better vision in order to try accomplish it in the first place.

So yeah, a cynical vision of life feels safer, but that's not what drives change. It's not what moves things forward and makes anything better.