In general agreed, but a quibble about:

Well, one possibility would be that a polyamorous inclination is simply rare. In that case, we wouldn't expect any society to adopt large-scale polyamorous practices for the same reason why we wouldn't expect any society to adopt e.g. large-scale asexual or homosexual practices.

I don't think I would necessarily expect that.

Of course, it depends a lot on what we mean by "large scale". A society with a 5% Buddhist minority can still have large-scale Buddhist practice (e.g., millions of people practicing Buddhism in a public and generally accepted way). I would say some U.S. states are visibly adopting homosexual practices (e.g. same-sex marriage) on a large scale, despite it being very much a minority option, for example.

In the absence of any other social forces pushing people towards nominally exclusive monogamy/monoandry (and heterosexuality, come to that) I would expect something like that kind of heterogeneity for natural inclinations.

Of course, in the real world those social forces exist and are powerful, so my expectations change accordingly.