I feel like the "Jobless and living with parents" pejorative is missing the mark. Jobless -- So what? If a country has figured out how to provide for its people w/o jobs, so what? For example if we had perfect AI-robot slaves we'd all be jobless. Would it be a problem? Maybe we should ask people why they want a job, maybe what they want to do doesnt require one? "Living with their parents" - Again, so what? Much of t…
But if you read a little about the price that japan (and this demographic in particular) has been paying and what they think about it, I'm pretty sure you won't find anyone asking "so what".
Your point on how USA (and other countries) should be making better use of their living arrangements is valid. I don't think it compares to the situation Japan or most deeply densed urbanized countries in Asia.