That study is a correlation with self reported satisfaction. The effect size is that a doubling of ad spend results in a 3% change in satisfaction. I struggled to find good numbers but it appears as if ad spending in the USA has been a more or less constant percentage of GDP growth.
Thus the only real conclusion you can draw from your argument is that any increase in unhappiness caused by the internet was caused by its associated GDP growth increasing ad spend per capita.
Personally, I do think advertising has become more damaging precisely due to the internet but good luck proving it.
> And a practical point on this topic is that the benefits of the internet are, in practice, fringe, even if freely available to everyone
Ok, nevermind. I can't take anything else you say seriously when you call the ways the the internet has improved people's lives "fringe". I take it you never tried to take a bus pre-internet? Drive a car across the cohntry? Or lookup any information? The internet's effects on people is so far from fringe that it has seeped into almost everything we do at a fundamental level. Perhaps because of that you can't see it.