Actually this is kinda my point:
Canada: Liberal + Conservative = 81.1% (90.5% with Bloc, 97.6% w/ NDP, )
Australia: Liberal + Labor = 84.8% (95.4% with National)
Germany: CDU + SPD = 56% (68% w/ AfD, 79.7% w/FDP, 89.4% w/ Left)
France: REM + LR = 71.8% (79.9% w/ MoDem, 85.4% w/ UAI, 90.8% w/SOC)
I can continue if needed...
These 4 countries have 4 different voting systems but what they have in common is a parliament where seats are assigned proportionately to votes (there's no winner take all). Canada and Australia seem to be the closest to America than the others. (Note that I used house numbers for all because they have larger sample sizes so there's a lower bias in distributions)
So it is pretty clear that IRV is ''not,, the defining feature here that causes multi-party systems, nor weaker coalitions. Though we'd have to get more into game theory about coalitions to talk more accurately and I want to stress that while I believe voting methods is a strong tool to prevent these things it isn't the only necessary tool (it's just the biggest step forward we can make given the current US system).
> (I purposefully avoided the common party names since they have connotations how Americans view politics that are extremely inaccurate).
I think this is fair. As I've argued elsewhere America's two party system is closer to a two coalition system. Which actually Australia is a better example of two coalition domination, IMO, than Canada (though I would say a close runner up).