E.g this article, written by a sexuality counselor, claims that some experts believe that there could be as many 15 million Americans swinging on a regular basis, and basically says that swinging works for some couples and doesn't work for others.

I don't believe that for a second and you should apply a little more critical thought to these numbers. What experts? What are they basing this on? Searching for this, I find nothing but echo chambers of media articles - "experts say", "some experts think", etc. Is 15 million remotely plausible? There are ~232m adults in the US, half are married, so 15m swingers would imply that 13% of marriages are open.

Slightly better are 'estimates' (or was it 'a study'?) attributed to the Kinsey Institute of 2-4% of married couples being swingers, but that's also quoted as '2-4m' (a bit different) and one commenter even quotes it as 2-4% being 'the BDSM and swing communities', which reduces the size even more. All irrelevant, since I am unable to track down any study or official statement from Kinsey so I can't even look at the methodology or assumptions they made to get those supposed numbers.