The last time we got a shake up in the two party system was with the civil war. Even then, we didn’t get three parties, we just replaced one party with another. 1912 was a notable but unrepeatable exception, but not an “upset”. We still elected one of the two major parties, and four years later it was back to Republicans and Democrats. It’s also notable that Taft and Roosevelt were both Republicans, so Roosevelt running as a Progressive meant that they split the vote and Democrats won with only 41.8% of the vote. Republicans were the left party at the time, so the left split the vote and got a conservative. Your exception shows exactly why third party runs are boneheaded.

Any third party that had the means to run a viable third party candidate would easily be capable of running an inside strategy to replace the Democratic establishment. Unlike the fantasy of a third party approach, that strategy has worked in the past. If there aren’t enough Democratic voters who are pissed enough at the Democratic establishment to do a takeover of the party, then there definitely aren’t enough to win a third party strategy.