Similar to comments above there's a difference between poly and open. I've not tried either but I've multiple good friends who are in "monogamish" relationships and it seems to work pretty well. For them the non-monogomy is just fun they have with others, but ultimately their partnership comes first. Otherwise it's very similar to the monogamy you describe but with agreed exceptions to sexual exclusivity.
It's not for everyone and it takes a lot communication (and low levels of jealousy) but it seems to work well at providing the structure and stability of marriage without forcing the full sexual exclusivity that some find constricting.
Why does their partnership come first? Whats stopping you from finding somebody better to make a priority? Isn’t that the point of being poly is to have the ability to shop around?
It takes work, for both people to compromise, to critically self-evaluate and improve.
When it's easy to just "shop around", you never really have to look hard in the mirror. It's easier to just internally assign blame to the other person: "they're not meeting my needs", and go off seeking someone else who will.
There's value in resilience, in building up your character so that you can endure turmoil.
All relationships have stormy times.
A key facet of emotional maturity is to be able to distinguish climate from weather.
It's not for everyone and it takes a lot communication (and low levels of jealousy) but it seems to work well at providing the structure and stability of marriage without forcing the full sexual exclusivity that some find constricting.