Earlier quoted context omitted.
It's also useful to note that the "nuclear family" is a very Western thing, and the US takes it to an extreme. In other parts of the world it's abnormal for adults in their 20s to be living anywhere but with their parents, often even with a fiance or new spouse in the mix. The idea that having a multi-generational household means that the younger members haven't "grown up" and need to "learn to live on their own" is…
While it's true this seems like a strong US thing, I haven't heard many happy long-term stories of folks living intergenerationally. In my experience (talking to people from a variety of countries), most couples in their 20s/30s want their own place (even in the countries where it's not common). It seems it can even lead to happier intergenerational relationships when the roomate-type fights don't need to happen. It…
On the first point, from what I've seen, there is certainly a lot of conflict if intergenerational couples stay together under the same roof for prolonged periods of time, whether it's the parents living with the kids or the other way around. For short periods of time, it definitely works out positively.
I would argue that it's less because of wealth or even expenses, and moreso because of cultural norms, that children tend to stay with their parents for longer in Europe. I don't know if this comes across as anecdotal, but my SO's parents often requested her to choose a medical university closer to home, and even suggested we stay with them, even though they are extremely affluent and comfortable. Even now, after living separately in another city, her parents and relatives often stop by to check up and stay for a couple of days very often. Of course, the young will always prefer to move apart to retain some privacy, especially once the kids arrive. Obviously there's the shadow of European expenses hanging over everyone's heads.