Possibly worth noting something about japanese culture again... Not sure where I was reading about this, but basically, in western culture, we perceive that we can drastically change our lives well into our twenties, and possibly even our 30s...

In Japan, many people feel locked into their life choices by the age of 25 or so, and its for this reason Japanese protagonists are so young mostly. Any older and they would be unable to be able to change so drastically. This makes even the younger adults easy to be cynics, even the hermit is quite cynical and fairly young.

I didn't know that about Trinity Soul, but it's worth noting that TS is non-canon, and other Persona games have featured adult Persona users--arguably, P4 has two adult Persona users, Namatame and Adachi, although Izanami's interference could be credited there. Kandori of P1 is another example that could be argued, but both halves of P2 clearly have adult Persona users.

Many of the adults that P3 spends significant time with are a part of SLs, and part of the point of SLs is that these people have issues to work through. By the end of the SL, many are at least making an effort to improve, just like the teenage SLs. Some of them may be more innately admirable or likeable than others--I find Bunkichi and Mitsuko to be a very likeable grandparent-like couple, whose continued grief over the loss of their son is understandable (note that it has not caused them to do anything neglectful or malicious toward others--Bunkichi didn't suddenly become an alcoholic or something). Yet none of them are cardboard paragons of virtue--that's what makes them interesting, three-dimensional characters.

As for stories at large, even in a well-adjusted home, no one is perfect, and the issues that they have to work through are where the drama comes from. No conflict, no story.

The overall plotline does seem to have a definite note of the children cleaning up the adults' mess, though. For the brief time that we meet him, Mitsuru's father hardly seems all bad, but he is also paying for what his father did.

The dorm does seem to have minimal supervision, though--perhaps the influence of Kirijo so that SEES can take care of its business? Perhaps the Chairman is supposed to be a greater presence but is bad at his job... *cough cough* It does not seem unreasonable to me, though, that cultural influence could be a factor here, though how much of a factor I couldn't say.

Orphan/amnesia/the-parent-who-gets-mentioned-once-and-is-never-seen-again is definitely troperific, and P3 loves its tropes. The teenage superhero seems to be very common in manga and anime, with parental presence at varying levels. Still, one aspect of its themes is death, so all of these dead parents fits into the themes as well.

The most well-adjusted character I can think of is Koro-chan, so maybe the message isn't "don't grow up," it's "dogs are the best."