I wasn't old enough to vote in 2015 but I almost staunchly supported the platform Labour stood on under Miliband. However, given that Labour was leading the Tories in most polls from January 2011 all the way until early 2015 but that they lost so decisively tells you that the pollsters and pundits can never really be accurate. The end of the Miliband era was also the end of the social democratic image the Labour Party had held so strongly since John Smith was elected leader in 1992.
Most people still believe the myth that Labour overspent whilst in government whilst making more irrational decisions that the Conservatives have done in the last six. They also blame Labour for the crash and for uncontrolled immigration soaring over the past few years. With the rise of UKIP as well as the Brexit vote, the party is losing its key demographic whilst pushing into its comfort zone by a lurch even further to the left under Corbyn and Mcdonnell. The party is now merely a fringe movement which has no appearance of or aspiration to be the alternative government the British people need. I would not immediately write Labour off but as a Blairite I would not give them a chance under such messy, nonsensical policies in a world shaping fast to be a catalyst for broad regimental change. If, however there was a change in leadership to a more centrist MP such as Dan Jarvis, Chuka Umunna, Keir Starmer, etc then I would seriously reconsider my opinion as it seems to be an internal battle between progression and tradition. 2020 will see Labour lose heavily to the Conservatives and personally I don't see them entering power until at least 2030 but what's amazing is that a swing of 10% (which gave Blair his landslide in 1997) will be needed in the future for JUST a working majority of three!