And the reality that Labour lost the economic message decisively in the 2015 campaign; whilst failing to win marginals in places like Croydon, which was hardly down to UKIP. In 2017, you at least had both leaders on equal personal footings by the end, combined with Labour winning the economic argument.
2015 was just a reminder that it's hard for an incumbent to lose unless something changes in the campaign.
If the Brexit issue does place electoral pressure on Labour, it won't be in places like that, I suspect.
And yes; the fetishization of Labour losing it's 'white working class vote' (a story, and theme which I believe has been popular since the 1930s) means that people are ignoring that all it takes is a modest shift/revival for the pro-EU parties, to cost Labour a number of target seats, or current marginals it holds.
To give an example, Ashfield is one of the Labour-Leave seats where the Lib Dems had 7,000 votes in 2015, and fell to 2,000- all it takes is some pissed off pro-Europeans to vote for them, and Labours vote falls. Of course this is just a random example I saw on twitter- but it's a fatal excuse to believe that as Dennis Healey said '[pro-Europeans] have nowhere else to go'.