I thought the major argument for polyamory was that some people are happier living that way.

Your first argument suggests that you find such arguments repulsive, but I can't see why.

The major argument for polyamory isn't that it's subversive, though I'm sure some people get a kick out of that.

However, I don't think polyamory is at all mainstream, even though it's closer than it was in past decades. Note that none of those accused in political sex scandals have said that they had an non-monogamous agreement with their spouse, >2 marriages are used as a slippery slope argument against homosexual marriage, and afaik, the only popular fiction about polyamory presents it as very unusual.