First - I am mono (surprise surprise
I think this is a very valid question and don't see a reason to get flamed for it.
I do believe that your point is valid. I also believe that a lot of people have struggled with non-monogamy before seeing it as a valid option and there is health and freedom in that...live and let live.
I do see your point however in that we often talk about "ultimatums" in the sense of if the mono partner says no to opening up the relationship or they will leave. That is an ultimatum or a consequence. BUT
looking at your partner and saying "I need to open up our relationship and want you included but if you can't follow I will have to move on to pursue this" is also an ultimatum or consequence.
I think there is already a lot of support for people trying to remain monogamous. Most marriage counselling/couples counselling is focussed on that in relation to affairs or lack of connection.
The question I have is "when someone is miserable or constantly struggling to survive in a mono/poly configuration, why do they hold on so long or so hard?" Is it the kids, the houses, the cars, the retirement plan, the public scrutiny at failing? I believe people can love someone so much they can get to a healthy place of compromise…but it has to be healthy. I see co-dependence where some people, whether mono or poly stay in relationships that cause them excessive grief, distraction, inability to focus and some to the point of needing drugs to control anxiety. That is not love…that is a lack of self esteem, self confidence, self respect and personal strength.
Some relationships were meant to be and are worth working through…others are simply based on the fear of being alone.