I've only ever met one single couple where true polyamory worked over the long term, and they just celebrated 20 years together. They are an odd awesome couple that are extremely well matched. [They don't have kids.]
I even worked for Kink[dot]com (NSFW) for 4 years and I've thrown hundreds of sex parties with hundreds of people going bonkers all night long (and often into the next day). I've tried it myself in a few relationships and I'm happy that people experiment with it.
But, I strongly doubt it'll ever be mainstream. I'm pretty convinced at this point that it'll always remain a fringe fantasy for the majority of the population. Especially outside of the bubble of certain very small parts of the world.