My understanding is the existence of selection does not necessarily mean every trait that exists right now has an evolutionary benefit. It is more coarse grained that anything that doesn’t prevent you from breeding is acceptable. Depressed people are not made infertile by their depression, so there will be a subset of depressed people (assuming depression even has a hereditary component). This doesn’t mean the trait of depression has an advantage in order to exist, it just isn’t so much of a disadvantage that it doesn’t exist.

Only the system as a whole must carry superior fitness, not each of its individual components. Given a sufficiently complex system, its rather expected that there will be negative, or even outright destructive functions that arise. You can certainly try to find a positive reason for why cancer, disease, death during conception, etc exist, but there is a much simpler explanation.

Depression in this view, isn't something outright that was adaptively constructed, but merely a side effect of how the mind works.

Patience, "biding our time", "hunkering down", etc. are actual emotional states we can experience and recognize, and depression doesn't feel even remotely like for most people as far as I'm aware.

Also, this explanation would only really work if the entire group entered a dormant depressed state together. Otherwise, the non-depressed ones would capitalize the tribe's resource and everyone would still end up screwed. But depression doesn't seem to have that sort contagious social component. On the contrary, when someone is depressed, the immediate response by people around them is generally try to "cheer them up" or encourage them to exit that depressed state. And while the depressed person is likely conveying a whole lot of negative sentiment, most aren't actively attempting to get the people around them to be depressed too. That's the last thing most depressed people want.

I remember from my days studying to be an actuary that the population that can best estimate mortality odds from the gut are actually the depressed. (Most of us tend to be way too optimistic about common risks and pessimistic about uncommon ones.)

This was also used to explain mammalian postpartum depression, when the mother has to make a wretching call as to whether to keep the offspring given its health, her health and the environmental context.

ChatGPT, asked to translate to a high schooler: "Basically, this study found that young people with depression sometimes struggle to break automatic habits, especially when they’re trying to avoid something bad and there isn’t a prize or reward for doing it."