Really, my opinion is, if you leave your marriage and find out you're mono... so what? There are plenty of mono people, and you wouldn't be thinking of leaving your marriage if it was perfect.
But my opinion on relationships is, the default is not to be in one. If I am in one, to stay in it I need to want it so much that I can't believe my luck to be in it. That I can't imagine my life without this person, no matter how many other people I have a chance to be with instead. That I would give years off of my life to be able to spend the remainder with this person.
That's how I deal with all my relationships. If it changes to "meh, it's alright" then the relationship is in danger. And if it can't be fixed, I'd rather be without that person (regardless of whether it means being alone or not).
But you seem to think of it in different terms. For me, if a relationship is neutral instead of good, it's not worth it. For you, it seems, if a relationship is neutral but not bad, or bad but only a little bit, it's still better than nothing.
I don't think so. Nothing is neutral. Neutral is the same as neutral, and a little bit bad is worse than neutral. And if that nothing has a chance to become something else, as it always does, then it becomes a positive, and therefore better than a neutral relationship.
And I'm of the opinion that there are tons of people you could be happy with. And I think there is no reason to prioritize someone just because they happen to be right here, as opposed to making you happy for instance.
I think in your case, I would probably break up even if I was sure I'd never meet someone else. But I can't be certain about that, because the idea of never meeting someone else seems incredibly ridiculous. I meet people pretty much every day.
Bottom line, in my opinion, is that you're in a relationship when you're not free to be yourself, to discover who "yourself" is. That's not a good relationship. Whether you are mono or poly, you need a relationship where you're on the same page with your partner(s) for things that important.