If promising some more progressive policy was a clear winner, why wouldn’t they do it? The answer I generally see implied or stated is that the dem establishment doesn’t want that policy, but that isn’t really an adequate explanation, because politicians are perfectly familiar with dishonesty. If supporting some progressive policy they didn’t like would win them power, they’d just promise it and then just not do that thing upon getting elected.

Because their personal motivations are not "maximize the chances for a Democratic win", but preserve the power of themselves and their allies with money and influence. If these policies become a centerpiece of the election and broadly popularized, it becomes dangerous to ignore it and advances the saliency regardless of the outcome, pushing it closer to someone actually doing it. A campaign that says "the rich are abusing workers to fill their pockets and the government should tax their wealth until there are no billionaires and provide benefits to the workers" is dangerous to the rich people, even if its initially proposed by someone with no intention of following through.