Third parties can't get a footing in America because we have legalized bribery and without corporate funding, you are not getting anywhere close to the Presidency. Ever.
People like to say "it's up to progressives to move the Democrats left" but the Democrats also have super delegates in order to basically automatically force any result they want. Even if you weren't a "Bernie Bro", the 2016 primary was very obviously rigged against him.
If we were going on sheer electability, people like him have actually stood in front of FOX News' crowds and convinced them that his points have merit. Because the actual truth about most "moderates" and even some "fiscal Conservatives" in America is that most of them have at least some vaguely progressive fiscal policy beliefs that the Democratic Party plays a part in dissuading the efficacy of, and you could easily convince them of at least some straight-up socialist policy.
How do you start a third party in America when the only way the current parties run is because they get literal hundreds of millions in donations on the regular in order to fund their campaigns and political parties.
Local work? Sure, local work makes a difference locally, but that doesn't translate to federal success, at all. Our two-party system didn't start because some Democrats and Republicans ran door to door since the last time a third party existed, the two-party problem existed in the fucking 1800s. Most third political parties got consolidated themselves, other political parties were more feasible because the cost of running elections were lower, and corporate consolidation was in its infancy in America. Also, lobbying (aka bribery) (officially) started in 1946.
The reality is that the two-party system isn't a bug, it's a feature of our system combined with late-stage capitalism. Companies own our political parties. While that doesn't mean every single person in a place of power is in some grand conspiracy, nothing is going to happen on that level that a number of large corporations do not explicitly approve of.
Speaking of, I love that Joe Biden got kicked, but if you're gonna say anyone got them kicked, it's also the capitalists lol. The biggest nail in the coffin was likely donators holding back their cash flow to put more pressure on Biden. Imagine a blossoming progressive party that was running on policies that make companies less money, and what would happen with the funding for that race.