I understand what you’re saying, I just don’t understand you. What strawman? I’m assuming that you’re happy in the center because that’s kind of how you’re talking. If you’re not happy in the center then ok. You and me consistently voting for centrists is not going to pull the democrats left.
The money (and the swing voter, apparently) is happiest with center-right policies, and if we vote “blue no matter who” that’s all we’ll get. So if the center (and the money) decide the elections, how will things ever move left? The minute some future dems start going left, they’ll lose the center, and with it the election.
There’s no way out of that dynamic without a lot of new voters to ensure victory for left policies, which I’m saying, dems are unlikely to attract with their center-right candidates and policy. Is this a chicken and the egg problem? Maybe, but it also seems like a lot of people are perfectly happy the way it is.