Like, let's create a simple model based on what I think are decently realistic assumptions.
Let's say we have 1000 people, and let's assume they're all monoamorous and also let's ignore sexual orientation to simplify the model.
There's a lot of single people but relationships tend to last a while and end at unpredictable times, and it's kind of a faux pas to move in on a person who's recently become single.
So let's optimistically say that only half of all people are currently in relationships. That leaves about 500.
And assuming you're straight, as lonely men (oh no! Look out, it's the Male Loneliness Epidemic!) often are, you're left with about 250. Let's say up to a fifth are too old or too young for you, not looking or not available or looking for something entirely incompatible with what you need and want, or just very much not into you.
You're left with 200 dating prospects out of 1000. I think that's a very optimistic number. And you have a lot of competition for anyone. Potentially up to 250, every other person of your gender that's also single.
Now let's assume only 20% of all people are open to polyamory, with the same parametres otherwise. Let's assume you are, as well.
Out of the 800 monoamorous people in that group, assuming still about half are currently in relationships, that's 400 people or so. The remaining 400 undergo the same winnowing down as before. You're left with 160 dating prospects from that group, and you have more competition, sure, but it's not by THAT much. Out of the mono-only people that's 200, and let's be pessimistic and say every single poly-friendly person regardless of their relationship status is looking, so that's 100 more. So you're competing against 300 people as opposed to 250.
But here's the thing. A lot of existing polycules will not be polysaturated. And they will branch out and grow, polycules often overlap, they're not closed off groups where everyone is in mutual relationships with everyone else. It's hard to polysaturate a group of 200 people completely.
Let's say (and I think I'm lowballing the number a lot, for several reasons) that out of the 80 or so people who are compatible with you in terms of age and needs, only about 30 are looking or open to a relationship.
So if monoamorous only, you have 200 potential partners and compete tooth-and-nail against about 250 people. And that's the best case scenario.
And if we introduce polyamory as something unusual but decently present, you have 190 potential partners and you're strictly competing against 200 only, with 100 more not really any big risk of "stealing" any potential partner away from you. And that's the worst case scenario.
And also, polycules change dynamically and every person who becomes available for dating or sex isn't immediately a potential target for the affection of literally every person who's single, which does happen in mononormative groups.