It is ahistorical to think a third party will win at all with FPTP. Libertarian Gary Johnson had over three times more votes than Stein in 2016 and he only got a bit past 3%. Neither of them had actual EC votes.
The government is not going to trend towards leftism from voting. It is going to trend towards fascism and authoritarianism because the Repubs reliably come out to vote and will vote for someone who already did a putsch and is openly talking about dictatorship and assassinating political opponents. The Dems are going to trend (further) towards fascism chasing the middle voter if they do get another chance.
Plenty of countries have managed to be authoritarian for decades if not longer, but perhaps the regime would be toppled after a period of war. I don't think there's any guarantee an improved government would form out of the rubble from that.