I don't know, Claire. I'm not sure you've made the case here for giving up on the existing system, just because so many people get divorced.
I mean, basically I do agree with you that lifelong monogamy and the nuclear family is a model that doesn't work all that well for many people these days. I also agree that polyamory in some form is probably a better option and needs to be a legally sanctioned option. I think we should also explore short-term marriage/cohabitation contracts.
But there are other reasons why marriage isn't working, and I think those factors need to be explored. One is economic--the fact that it's so hard for two people to earn enough to live on and provide for children, while also doing the physical labor of cooking, cleaning, supervising play, changing diapers, etc. Another is that the emotional needs of the adults and the children are unlikely to be met consistently in such a tiny social unit. Families worked better when they were bigger and involved multiple generations.
Finally, the way we do divorce in the US works for absolutely no one but the lawyers. It shouldn't have to be so universally awful, but it is handled by a legal system that assumes the best solutions will be reached by an adversarial process.
Once again, I do agree with your position, but I think you oversimplify the issue.