It certainly does seem to be the case that people aren’t getting getting more mentally healthy(or even staying even) at a population level; but that only tells you that they aren’t getting better at mental health if you can safely treat the circumstances as constant.
If the adversity of the circumstances is increasing a population whose coping techniques are improving might still only be treading water or actually declining in terms of results, depending on which thing is happening faster. The reverse could also occur, though finding an environment where psychological pressure is receding at a population level fast enough to keep steadily more brittle people at a constant level of adjustedness is probably less common in practice.
I’m not certain how one would disentangle level of environmental pressure from familiarity with coping technique(since people’s coping abilities presumably cloud their reports of how much pressure is present); but I’d suspect that there may well be some of this going on because of the number of contexts where people are judged vs. their peers in situations where loss of social status (and often good old fashioned grinding penury) are the known consequences for the ones who fall toward the bottom of the distribution.
In work or school, say, you aren’t being judged against a static target that improved coping mechanisms would allow you to hit with gradually increasing levels of wellbeing; but relative to the performance of your peers. If they are getting better at coping your performance will fall behind in relative terms(which imposes its own set of stresses); and if you and they both improving you’ll still have to use your ability to take on more pressure on order to do more in order to at least see stay where you were in relative terms.
Unless there’s something(whether it’s just custom, active pushback from unions or the like) keeping the exercise from being an open-ended judgement against peer performance without reference to some static standard of adequacy people are likely to end up with a similar outcome even if their skills improve because they are induced to take on more pressure if they can handle it.
