Okay, now that that's out of the way.
What's that? Pre-60's, roughly half of adults age 18-29 lived with their parents? Musta been something wrong with those pre WW2 adults. Probably because pre-WW2 adults were too busy playing with toys like... Jet skis, motorcycles, unnecessarily powerful cars, colour televisions, film cameras, going to discotheques and theatres...
Oh, actually, it was just that pre-WW2 generational living was extremely common because of small communities and difficult economic situations, which are still extremely common across most economically struggling cultures. It wasn't until the industrial boom of WW2 in the US that we were so flooded with toys and goods... for people of all ages. Adults have been buying excess and ridiculous amounts of novelty goods and toys since at least the 60's, which was one of the few times where the US budget has been in the positive. Beginning with this new way a living, a sharp economic spiral can be seen starting with, quite literally, the boomers. (https://usafacts.org/state-of-the-union/economy/
Now, its interesting that you couple Sam Vaknin's idea of "child adultification", or Precocious Adulthood Syndrome, as not only does Sam's purported outcome go against the typical assessment of outcomes created by an emotionally incestuous relationship, Sam also commonly mentions that another outcome of PrAs is one that does
coincide with the typically assessed outcomes of emotionally incestuous relationships. That being, compensatory infantalism (Peter Pan Syndrom, a.k.a. Failure to Launch) versus the early onset of maturation coupled with an uptake of narcissistic and asocial traits. Essentially, Sam says that when children experience adultification they may act like children later on to compensate, but they might also take on more responsibility than they should as they feel the need to people please.
Well, that's not a very definitive answer. Typically I would say that such puts you on the right course, but as Sam, a clinically diagnosed narcissist, has a history of claiming that they were the most prominent figure in developing terminology and therapy strategies in the field of narcissist personality disorder research, its strange that he takes such a lukewarm approach to child adultification.
Perhaps it is because child adultification was already a field present before Sam began his work in such. The term generally refers to an improper biasing of adult like characteristics onto a child; e.g. assuming that when a child makes a mistake, such as a social taboo, they should know better, as an adult does, and thus punish a child based an a metric unsuitable for their experience and mental maturity. This, typically, can lead to outcomes such as ODD (oppositional defiant disorder). More often than not, adultification of children has less to do with maturation and more to do with deviancy disorders and crime statistics.
So, its difficult to see how we can correlate any of these data sets into a "modern emergence" in any facet. Realistically, this is the late-stage effect of what has been in motion since the post-boom industry of WW2. A settling goods-based economic bubble in a massively wealthy time of emerging industry has led to disillusion about how society, as a whole, functions. Those who are alive no longer know what a pre-WW2 society and economy looks like, thus it seems like a downward trending economy and a dissolving society, when, in reality, things are more like the pre-WW2 world now than they ever were. Current generations attempt to push back against this disillusionment by recklessly partaking in the same goods that ever generation has, in an attempt to stave off the regression to a more normative way of existing. Much as a drunk or a gambler do when they're on their last leg and have nothing left to lose, they throw more than they have into the final pot in hopes of disillusioning themselves into believing that things will stay they way they are forever, in light of accepting failure and working around their circumstances.