Fundamentally, marriage is a patriarchal institution (and who wants to go to an institution 😉). Prior to the advent of agriculture, mating looked a lot different. That doesn’t mean that people can’t choose to commit to each other in that way, but it does say a lot about why wife is considered the most important role for a woman, other than mother (and you need one before the other in a patriarchy). Also you’ve got to get men to buy into the importance of marriage too if you are going to have that set up. As you’ve pointed out, being monogamously committed for life doesn’t guarantee anything, so yes, it would be great to have some other terms.
I have one life partner that I’m married to, and one that I’m not. A while back I did a crowd-sourcing for a better term than partner and we came up with amorido/amorida. It’s not like I use that often, because no-one else knows what that is, although if they speak Spanish, they might figure it out. But at least it’s a start.