To be clear, I wasn't criticizing the plural relationships in SoS; they all seem (to me) like "what the characters would do". (Except that the Juri/Miki/Kaitlyn thing kinda came out of left field for me, but I haven't seen most of RGU so I don't have a solid sense of Juri in particular. It didn't seem out of character, just unexpected, if that makes sense?)

In real life I suspect the prevalence of long chains is tied to the lack of general social acceptance for polyfidelity, which pushes the people who are into that sort of thing into a subculture. So you, say, meet someone on an online dating site who is into that, decide to give it a try, they introduce you to their friends, and now you are socializing with lots of people who are also into that. (This is approximately what happened to me.) If it were considered more normal, it might be more likely to be something that Alice and Bob and Carol decide to do just the three of them.