I think that's pretty clearly right - if you truly have good reasons to expect that the conditions for happiness are no longer possible, or are exceedingly unlikely, depression is absolutely a rational response. Conversely, quite a few people who are mostly happy are happy largely for irrational reasons and would be less so if their delusional positive beliefs were dispelled.
I guess the question is, though, what's your point? I don't think too many people would argue that depression is literally always irrational, even for someone imprisoned for life in solitary confinement at ADX Florence. So the way your thread is reading is as an intellectual defense of your ongoing depressive episode. That's a bit shakier, because it seems to resolve to an argument that depression is the most rational possible response to long-term negative trends like climate change. Most of us think it's possible to be rationally satisfied with life even if the future is bleak, and that's the point you're getting the most pushback on.
I used to intellectualize my own depression in the same terms, but I did finally realize that it's just my individual response, and that it's possible to be perfectly rational without that pervasive sense of despair. This hasn't made me much less depressed so far, granted. But I recognize that it's my own personal issue, and the justification in terms of bleak global trends is more of an effect of my depression than a cause of it.