If this were an upward spiral it couldn't go on infinitely (due to the width it would be impossible, at least in the real world), implying there's a maximum potential you can reach. But the bottom is infinitely far down.
Eventually the downward spiral would grow so large that it might collapse under its own weight. Maybe they snap and rebound with all that pent up energy.
That's interesting; I always pictured a downward spiral as spiralling inwards, sort of reminiscent of water draining from a sink or bath. Or like those weird spiral things you roll change around in?
I agree, but in this specific case you logically would have started in the centre of the spiral in the overhead view which would equate to downward movement in the side view.
But they definitely chose this because inverting it would absolutely be more associated with a downward spiral. And this kind of looks like a nice tree.