Again, you're intuitively correct. I actually agree with what you're saying, and I acknowledge that the current landscape is changing the fundamentals in a way that we can't fully understand just yet. But from an institutional perspective, there's going to be a bias against unproven strategies until evidence emerges that the risk of backfire is low. It's more "the devil you know" and all that. The testing ground for the electoral effectiveness of progressive voter mobilization is downballot. Until consistent signs of success emerge in downballot races, the Party is simply not going to take a risk on the national stage. And I'd counter that if downballot success isn't happening, then there's something underneath your argument that might be missing. If there's some kind of underlying fundamental that's missing from the puzzle, it might be phrased like this (I'm reworking your last point):

In the current polarized environment, mobilizing one’s own base ~~is~~ should be more effective than appealing to the center, so much so that even if you’re promoting broadly unpopular policies, it can still win against someone who has failed to adapt. But young and progressive voter turnout is consistently low enough that this positive effect is dampened.

I'll admit it's a chicken-and-egg argument, in that you can't test progressive mobilization without first putting forward a progressive candidate, which isn't going to happen until progressive voters mobilize, and so on. But I think the Party's major, overwhelming fear is that progressive voters won't show up even if you give them what they want, and then the electoral damage would be overwhelming. To put a bit of punctuation on it, my state (North Carolina) has a persistent Republican supermajority in the Legislature which many locals are tying directly to the leftward shift of the party at the national level. The more leftward the Democratic Party goes on social/cultural issues, the redder North Carolina gets, especially in the past few years. We had a Democratic trifecta as recently as 2010 and they've so thoroughly baked in Republican control that I don't anticipate Democrats taking control of either house (or the judiciary) through the end of my life, which is crippling for centrists and progressives of all stripes.