Correct, what i showed disproved Duverger's law by counter-example. So how long ago it happened is irrelevant to that end.
As to why we havent seen a switch in the last 100 years is simple... because people bought the lie about a two-party system that was used to ensure evil remained in power, and now we have schmucks like you who want to do good but instead do evil because they bought into it (along with most americans).
It will only be true for so long as you buy the lie.