I have said it before, so I will say it again: only a third of voters are core Labour voters.
In order to win a general election the Labour Party needs to persuade a lot of floating voters and some who would be soft Tories or Lib Dems to vote Labour. This mainly means reducing what they perceive as risks. The other side will play up those risks. The media will make sure of that.
Winning an election is about the middle ground, it always is. UNLESS, the third of people who never vote actually turned out and voted. That would change the game entirely. Yet, Labour has not invested in the level of community campaigning that would be necessary to make that happen. It is all high level stuff with billboards and lots of internal meetings. So its back to trying to persuade the middle ground to vote Labour and that probably means losing next time and changing leader.