The rest of these posts focuses on how insignificant the third parties are, however the real reason is that our democracy was designed as an adversarial democracy, it’s been designed so that the two major parties, whatever those two major parties are, are supposed to be in a balance of compromise while also locked into a battle for control. The theory behind this is basically that through the adversarial process a measure of balance would be achieved.
Obviously the design is broken as we’re on the road to a second civil war, however that too might be by design as Jefferson, one of the architects of our democracy, can be quoted as saying “The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants” so maybe they just saw revolution/civil war as a natural occurrence in their design.
As for third party replacements, the Republicans, if they lose and keep bowing to trump and losing popularity, are facing the chance to be replaced by another political party, most likely it will be the Democrats that become the conservative party again, as they were prior to the civil rights movement and the Southern Strategy , as many neocon Republicans have left their now far right party to join the much more moderate democrats, so in theory a new Leftist/Liberal party would arise to take their place. So to achieve that first the Republican party would need to continue it’s downward spiral, and then a third party would need to pass a certain threshold for votes, once that’s done we would have our new two parties.
It also doesn’t help that the current most popular third parties, Green and Libertarian, are both heavily funded by both Right wing and foreign sources, so even their marginal successes currently are dubious.
Now with this all said I’m sure someone who’s actually got law degrees or political science degrees could explain this better and more accurately but this is as I understand it, overall we have a lot of hurdles to overcome but until the Republican party crumbles away we’re stuck with a party that has turned into the Qult of Donald Trump who wants to end democracy completely so that they can have all of the power and control, and that’s why we must not focus on third parties but instead focus on beating the wannabe tyrants that want to turn our country into the Trump kingdom.