But a third party will likely never be viable in within the lifetimes of people alive today, unless the Democrats suddenly decide to overturn FPTP voting.

So understand what you are asking of people. People are more than "frustrated with the lack of progress", they are enraged because they recognise that the current system will NEVER deliver them real justice and dignity. That they will be faced with this exact same situation every single election. And you are asking them to be content living with their rights and well-being on a knife-edge, likely for the rest of their lives. Because while the Democrats won't give them justice, the alternative is fascism.

So you are correct, the Republicans are objectively worse, and people should vote for the most progressive viable candidates possible. But the neoliberal tendency to demand that leftists stop complaining while they give up everything in the name of "compromise", and then tendency to blame leftists for neoliberal losses anyway, is galling.