I think when practiced mindfully, this kind of relationship structure is really powerful. You don't have to go through things alone or with only one other person who'll always truly have your back.
I don't want to idealize it: polyamory is hard work, without which it becomes a toxic dumpsterfire shitshow. Being able to have a tight knit community like this and maintain it for years is tough, and requires lots of patience, honesty, vulnerability and mindfulness. But it's gone a long way towards showing me how flimsy the traditional family structure in the West can be -- particularly for those of us who have complicated relationships with the families we were born into.
To me as a late-30s person with no intention to reproduce, that seems a little like: damned if you do, damned if you don't. And, it leaves me wondering if I've missed out by not pursuing this free love thing.
What I'm hearing from the parent comment is that they're weighing positives and negatives of focusing on commitment vs focusing on freedom, respectively, in different situations.
Maybe I'm weird, or doing it wrong, but in my life so far, I just don't see a real tie between romantic relationships and those familial/aging/life responsibilities. My partner and I each have our webs of responsibilities; the edge between us goes both ways, and I can't see how it being labelled "romantic partner" has any bearing on the rest of the web. Sometimes that edge makes dealing with a particular family situation easier, sometimes it doesn't. I don't think it's even possible to say whether there's any net effect, since of course there's more to it than this, and the experiment only gets run once.
There are two straightforward contributors to younger people being more interested in that sort of romantic fluidity: hormones and energy.