> I became a citizen of the US in 2011, but I still don't understand why Americans think they can only have a viable 3rd party with ranked choice voting.

You don't need IRV (sometimes, annoyingly, called Ranked Choice Voting), you just need any of a wide range of electoral systems that aren't the ones usually used in the US for federal and state legislative and executive elections.

> The response I hear all the time is "We live in a winner take all system." Yeah, so does everyone else. All politics is zero-sum, including in every other country.

First, this is wrong on its face. Except in a structural duopoly, politics very often is not zero-sum, and, in any case, “winner take all” is not the same thing as zero sum, it's both zero sum and all or nothing. Even zero-sum political systems need not be all-or-nothing.

But, more important, you are entirely misunderstanding the meaning of the response. It's not about the overall system being “winner take all” (which, even in the US, it is not) or zero-sum (which is arguably a consequence of what is being discussed, but not the direct issue.) “Winner-take-all” is a term for either Plurality or Majority/runoff election systems, which structurally make (more strongly in the first case) the expected value of voting for a candidate other than the most favored of the top two negative, creating a structural incentive for at least local duopoly when used for legislative districts or regional executive seats, and for national duopoly when used for national office.

While the national duopoly is frequently commented on, it's actually historical weaker than local duopoly in the US. You can find cases where locally (even for a while at the state level, most notably the Minnesota Farmer-Labor Party before they merged with the Democrats) one of the two parties in the duopoly isn't a major national party, for instance.

> Other countries have viable 3rd parties and when no one has a majority you build a coalition.

Name one country that does so with a strong, separately elected (not by the legislature) executive who is elected (directly or indirectly) by a method which doesn't involve ranked choices and uses single-member national legislative districts with majority/runoff or plurality elections.

The UK manages it with plurality legislative constituencies, largely because of regional differences (much like the US has sometimes had regionally differing duopoly, but with much more significance nationally because it's a parliamentary system so parties that aren't nationally top-two that get seats can still have a role in choosing and participating in the national executive other than by endorsing a major party candidate in advance and surrendering power to them.)

> I really fail to see why Americans insist they can't have a viable 3rd party other than sheer historical prejudice, ignorance, and inertia.

Because some of us have actually studied comparative government and the structural incentives and effects of different electoral systems.

> My suspicion is that ranked choice voting isn't going to fix anything in the US.

It probably won't; it's the alternative electoral system that does the absolute least to resolve the strucural impediments created by the systems currently in use (it's basically a small step less strong on those problems than majority/runoff), and unless it existed at both stages of the Presidential election (or that was reduced to a single-stage popular vote using RCV/IRV), it wouldn't address one of the biggest reinforcers of national duopoly.

> The US's problems are 90% cultural and institutional.

The duopoly problem is 100% institutional and 0% cultural, in that it is entirely an effect of the legal and structural design of the institutions of government, especially the electoral systems in use, and not at all a separate product of culture.