There's a fair amount of y'all talking past each other in this exchange. Just, FYI.
I think there are structural/historical reasons and dumb luck that blunt some of b_t's criticisms. Aaron Burr sticking us with the filibuster, and slavery/Jim Crow/racism entrenching it favors the current GOP greatly. So does the timing of SC vacancies which has allowed Clinton/Obama/Biden to only fill 4 seats in 18 years while Bush/Trump filled 5 seats in 12 years. You can quibble about Dem effectiveness in filling Scalia's seat, but no matter your argument, it's still a 5-4 court overturning Roe, ruling for Citizens United, gutting the VRA, and all the rest of their bullshit.
But he's absolutely right that Dems have simply written off too much of the electorate to be considered good at politics. Many of them did so because of personal greed; the path to riches doesn't lie in passing laws that help poor people financially (see Sinema, but she's only an obvious example of a legion of her type that fly under the radar). But many of them do so because of cowardice or a total inability to understand what wins elections. b_t nailed this. Does the average voter who is persuadable to 1. vote to begin with or 2. vote Dem over other options care about gerrymandering (if they even understand it)? Care who sits on the Supreme Court? Care that trans kids are prone to suicide, that blacks are more likely to suffer from our brutalizing police forces, that women make less than men, etc.? Of course they don't. They want to know if you have any ideas that might improve THEIR lives. Speak to them, or don't be so surprised that the carnival barker who does make them promises wins their votes.