Once a society attains a certain level of efficiency or productivity, changing social structures can free up significant amounts of otherwise untapped potential. Every modern industrial society had a rather rigid concept of "women's work" until relatively recently. The technological advances (and immigration) that broke this tradition resulted in a tremendous increase in human capital and significant economic growth (among many other mostly-but-not-entirely good things).

Modern polygynous societies are vastly different from modern monogamous societies in ways that do not revolve around mono vs poly. Furthermore, I don't think many societies have been tolerant of polyamory, as opposed to polygamy. Given that other values (having kids, working, buying needless crap) remain relatively constant, polyamory would likely help revive the strong social support networks of yesteryear and exhibit positive returns to scale versus the current system. This is not to say it would definitely result in an improvement, but demonstrating, "Polygynous societies aren't that productive, therefore monogamous norms are vital to continued economic success" requires vastly stronger evidence than you cite.