I've always thought there was a word missing in polyamory. It really ought to be serial polyamory, just as some people fashionably discuss marriage and divorce as serial monogamy. Humans evolved an imperfect form of monogamy meant chiefly to support the female and child in the first few years after birth when she did less hunting and gathering in order to support a new child. A male might stray during that time but his primary responsibility was providing the necessities of life for his progeny through a social contract with the child's mother. That imperfect monogamy still exists and may be more hard coded in some more than others. So in the context of poly relationships today it seems to me that a 3 to 5 year poly relationship should be the most we think of. It seems quite natural to me that poly relationships might fall apart after this and the players would be then free to start the cycle again.