I reject the premise of the question, Labour didn't lose on Thursday because they were too left wing - policies were pretty much irrelevant for this election (no-one can possibly believe the Tory manifesto), imo they ultimately lost because of three reasons:
1) Voters thought David Cameron would make a better leader of the country than Ed Miliband.
2) People actually believed that Labour overspending and not the global financial crisis caused the UK's debt and deficit problem. Labour should have been much more vocal about defending their economic record.
3) Voters thought Labour would do a deal with the SNP despite Ed ruling it out during the leader's debate.
The Tory propaganda machine was the difference here - you have the Sun, the Telegraph and the Daily Mail just repeating the same big lies again and again and people believe it. I dunno what the solution is tbh - do a deal with Murdoch like Tony Blair did I guess. Not a perfect solution but at least Murdoch didn't have it all his own way after Blair shagged his wife