Well that's absolutely where New Labour poisoned the well. There was no internal party management, there was just colossal complacency and neglect. People weren't involved, weren't given the chance to buy into the project - they were just offered nominal success, told not to quibble, and if they didn't like it they were no loss because they would never, ever vote Tory where as the centre ground New Labour was courting absolutely would.
Now all of that process is wrong, but the idea that the membership has to moderate it's expectations of its representatives - who are almost be default more left wing than they are - is unavoidable. The goal is to do that in a way that is inclusive of the membership rather than exploitative or neglectful.
This is where we come to Corbyn's election campaign, a campaign that burned pragmatists and moderates in favour of renewing the connection with the membership at the expense of everything else. You are right to say that Harman is not the opposite of Corbyn but you were also right when you said that a mistake of presentation on her part was used (exploited) as a campaign tool that drew contrast with Corbyn's 'no politics' approach. He took the hardline position and left his supporters to make the argument that Brownite/Moderate/soft left Labour supported Tory welfare policy.
His 'outsider' campaign setup a two speed race - no compromise vs too much compromise and the wheels inevitably came off both campaigns. But it's all the moderates fault for not supporting him, he's blameless.