I find myself in poly situation, I would start looking for a new house, new spouse and reasonable priced assassin, of course.
Seriously though, only form of poly I might be okey with is closed triad sort of deal. Time and attention are limited resources but depending on what jobs, hobbies and other obligations there are I can see three or fore adults be able to create staple and happy relationship. That’s still small enough number of people that everyone can realistically know each other intimately and take everybody’s input when making plans. I do know some married people who also have very important long time friendships. These people do take each other account when planning for the future, see each other very often, look after each other, help out financially, are there for health issues and even have big arguments once in while. If my polyrelationship would be something like that but with sex and living together, then I might be able to deal with it and be happy. If there is respect, frank communication and everyone is very family centric and emotionally staple type, a small group poly relationship could work out. I think it’s unlikely and lot of work but doable under right circumstances. Still definitely not for everyone and I doubt I’m one those people. It’s hard enough to maintain relationship, balance finances, share shores and not to have conflicting long term goals with just two people.