I can't reply to you anymore, but the study you cited is at least a couple of steps removed from the discussion. First, it's about offloading navigation to a computer, and observing that humans use less of their navigation skills when doing so. This is very far removed from "smartphone use causes mental health problems".

Second, you claim it shows a "provably weaker hippocampus". But the study doesn't show that at all. It shows less activity in the hippocampus, which would be entirely expected, much like if we offloaded translation to a computer, we wouldn't see the same level of activity in the language centers of the brain.

The researchers themselves only conclude this from their study:

> These results help shape models of how hippocampal and prefrontal regions support navigation, planning and future simulation.

That's it.

You asked for a study supporting our over-use of phones in general is a net negative.

I gave a specific example that is well studied that could allow us to make pretty good guesses about other apps that do our thinking for us, which could explain the types of results we get in these studies.