I think intimacy/romance is self defined.
I don't see that. I'd ask the OP what he means by intimacy-- sex, romance, or emotional intimacy, continued deep feelings of friendship, respect, support?
All of what I'm talking about is directly related to this OP.
I don't see that. I see you making some generalizations, using unclear terms like "dead marriage." I am a student of history, so I take a larger view of the benefits of marriage. It could just be a legal arrangement. It could be a means to reproduce (have kids). It could be to get a mate who shares your career interests, say, a farmer who wants a wife who also has farming skills, or a soldier who needs a wife who will be okay with his absences and risks to his life in battle.
It's only in extremely recent times that marriages have become this romantic ideal, where you love and desire your spouse and make the (silly) promise to love and desire them forever, the same way you do on your wedding day. Obviously, no one can predict the future. If you marry at age 22, you will be a different person at 44 and may no longer love or desire, in the same way, or share interests with your also-maturing spouse.
Therefore, in my opinion, a marriage is not
automatically "dead," or no longer a marriage, if the initial "love," excitement, NRE, or shared interests wane. You can still deeply love your spouse if some emotional intimacy is there. If you're not putting in the work of keeping the relationship alive, because you got tired of trying, say, or you've just been basically avoiding the person you live with, if you've become indifferent, or there is hatred, abuse, etc., THEN I would say the marriage is "dead." In that case, if you opened the relationship, it would not be polyamory, because any kind of love for the spouse is gone, and you're just maintaining the very mere appearance of a marriage. If you dated one other person, you wouldn't be poly. You'd probably just be looking for a "soft landing" prior to breaking up. If, however, you dated more than one person, and planned to continue doing so, then you would be practicing polyamory.
Example: famous actor Spencer Tracy had a famously deep relationship with famous actor Katharine Hepburn, but they couldn't marry because Tracy's wife was a Roman Catholic and refused to divorce him, despite his adultery. He was married to his wife in name only. He wasn't polyamorous. He was monogamous with someone who was not his legal wife.
There are all kinds of marriages. I'd be careful which ones we call "dead."