In this case there will be plenty of women who already have their BB and will just be after lots of AF. This bodes perfectly for plate spinning, and if poly is mainstream you get a lot more women up for this. They won't be concerned about finding BB and will be out looking for AF even after they're in a relationship. This perfection for those who want to spin plates.
This is kind of what my experience entailed. The women were only interested in finding AFs. The problem with this was that most of the women were chasing after the same top 20% (or whatever) AFs, if they could be bothered to chase at all. Didn't matter if the field of "competitors" might actually be comprised of the top 20% of guys from the general population - they all just adjusted their sights accordingly, sometimes to an extent that they became totally unrealistic (and, therefore, were content to just go home with their "lower SMV" AF/BB, or to just focus on fucking other women, which was also a very common "plan-B" practice). Which meant that you really had to stand out if you actually wanted to spin a plate.
So I think monogamous relationships will be more stable if polyamory is mainstream, and finding such a relationship would still be easy because I think plenty of women out there do want monogamy.
I would say that a certain type of "polyamory" is already mainstream - it's the hookup culture.
It's actually kind of funny - I got into a debate with a female friend of mine regarding my "polyamorous" lifestyle once. She (like has been expressed many times here and elsewhere) couldn't figure out the attraction. She seemed to think it was "disgusting", too, though she didn't use that term.
"Aren't you afraid of catching diseases? Aren't the people you are hooking up with trashy and "slutty"? Isn't the whole thing kind of "degrading" (I can't think of her words, but it was that sort of view)?" and so on with her logic…
Except, as I pointed out, she held none of that disdain for all her single friends who were currently out there sleeping around and generally "slutting it up" (I use "slutting it up" as a colloquialism and not as a moral judgement). Because they were doing the exact same thing with the exact same kinds of people, but she didn't feel about them their activities the way she felt about mine.
And she couldn't answer that. She just said it "felt different" when it came to me. What was normal sexual behavior for single people became somehow aberrant and "disgusting" once someone was in a relationship.
So, yes, it could be more "mainstream" for people already in committed relationships, but it's pretty well accepted for everyone else.
If anything I can imagine those monogamy wanting women fighting for the best men because most of the men will be trying to get involved in some kind of poly dynamic.
Except that, given what I think I saw with the 80/20 rule, there are still going to be a lot of men who are going to feel left out of the sex game, so finding a monogamous partner may not be all that hard. I mean, that's kind of how it works today. The truly top quality guys can, if they so choose, remain "polyamorous" (i.e., spin plates, or have multiple sex partners even in a committed relationship), but there are a lot of still-high-quality-but-not-top-quality guys who won't have that as a choice, and they will be eager for monogamy.