Lots of first past the post systems maintain multiple parties, to a degree, Canada, Great Britain are the ones I'm most familiar with. They still tend toward a (sometimes shifting) dominant pair, with third party voters becoming strategic around those pairs in close constituencies. 1/

It's Duverger's tendency more than "law", but even where systems don't collapse to two-party-ness, strategic voting to avoid spoiling becomes a very dominant concern, limiting expressiveness. 2/

Proportional-representation-based systems permit voters to express their own preferences (rather than strategically managing a party landscape) much more, whether or not there's outright collapse to two parties or just weird first-past-the-post strategic voting to navigate. /fin