The reality of American political process is that it takes at least a billion dollars to run a Presidential campaign. (Thanks, SCOTUS) That kind of money doesn't come from unions, social activists, or proletariat donors. It comes from corporations and billionaires, and those people don't like revolution.
Until someone can demonstrate that you can get more votes with progressive, worker-friendly policy proposals than with a well funded propaganda machine, the DNC is going to keep chasing the less conservative billionaires. And no third party will even be relevant.
Money doesn't win the election, it's more of an entrance fee, and campaign financing is more complicated than just 'the campaign.' You have to account for PACs, party, and all the free messaging from sympathetic media outlets. Bernie pinned his hopes on going viral on social media, and mostly demonstrated that it's not a viable strategy, at least at the Presidential level. Might work OK for smaller races, like AOC, in a geographically small, relatively young district, but not nationally. Most people actively avoid political messaging, which is a fundamental problem if you plan to rely on organic distribution of a political message through social media. Especially social media controlled by billionaires that might be hostile to messages like 'billionaires bad, unions good.'