If one cannot get rid of depression then one cannot get rid of heart disease or cancer.
Many people who haven't been stuck in that positive feedback loop have still been blue. Their capacity to feel happiness is muted just like ours, but they are successfully able to get out of that with a behavioral change. That's not always the case with a depressive disorder, and it can take a multi-faceted approach of medications, lifestyle change, sleep hygiene improvement, sometimes electroshock therapy for those people to get out of it.
Pretty sure there are many distinct conditions that we now call "depression".
Your description is very true for some of them, and not at all for others.
Some people might get a single episode, then get through it one way or another, and never have another one.
Others might have a lifelong tendency or vulnerability to it, and in that sense it might always be there in the background. I'm fairly certain I was more or less depressed (although not diagnosed) in my teenage years and early 20's. In my later 20's things were brighter, although I probably had a tendency for some amount of depression, especially during the darker winter months of the year (I live in the north). However, there were definitely times when I wouldn't have considered myself depressed. I'm not sure I'd even say it was "there" at those times, in the sense of being actively present in any way, even in a non-dominant way.
Later on, I got another deeper episode of depression that I'm still battling.
Yet other people might have it always there in some way, as you say.
I struggle with similar feelings, and the idea that depression becomes something that is there but can be put to positive use absolutely tracks with my own experience.