If this were a unique poll finding, or the overall picture was mixed, I'd be relaxed about it. But its all part of a larger picture that's emerging. There's a few more polls & studies coming out suggesting that Corbyn would worsen Labour's chances among non-Labour voters and that, yes, he's the most divisive (
1,
2), saying that his outlook is out of kilter with the general population (
link), also saying that his policy approach is currently unpopular (
1,
2,
3,
4,
5,
6) and electorally speaking, primarily targeting the left, the young or non-voters alone won't work (
1,
2,
3,
4,
5). Also, that lingering claim that there was no appreciable difference between the Tories and Labour isn't supported either (
link). People knew full well what Labour stood for, and just didn't agree with it.
Also, that 5 million lost votes thing was studied (
link 1,
2) post-2010. Most of them died and were replaced by new voters and many left the party before 2010 but came back, so it not totally straight forward. However, in a nutshell
"The pool of left-wing defectors is just 400,000. They are outnumbered by more than six-to-one by the 2.6m defectors who do not place themselves to the left.". So tbh, doesn't really support the Corbyn move. Obviously we may see new studies emerging that updates or changes that picture.
People can change of course, this is just a snapshot of how things were between May & August 2015, or back in 2012. Corbyn's goal is to make a change in the future. However be under no illusions that he faces an absolutely mammoth task, and I see no reason to think that he's more likely to makes things better than worse.