> France has a similar presidential system. It's a purely cultural thing.
But France doesn't use simple one-round FPTP for either the Presidential or legislative elections, and doesn't combine the elections, either, using a two-rond majority-runoff system for each election (with additionally what amounts to a minimum threshold by eligible electorate size, rather than just share of turnout, for legislative seats for first-round wins.) Majority-runoff has a far less strong push to duopoly thaneither simple FPTP or the US Presidential election system.
It's not purely a cultural thing; to the extent it's cultural, it's largely cultural adaptation to structural incentives.
> You could argue the duopoly prevents extremism by straightjacking the political options, but after the Tea Party takeover the GOP and now Trump, that has been thoroughly debunked.
I wouldn't have argued that in the first place for other reasons, but Trump would, at worst, only debunk an absurdly absolutist version of that; electoral systems may structurally disfavor certain outcomes but they rarely absolutely prevent any particular ideological outcome.