Brexit was not the reason the “red wall” fell in 2019. Brexit may have been the excuse many lifelong Labour voters needed to finally take the plunge and vote Tory, but the root causes behind so many seats switching to the Conservatives are older than Brexit (and were also present in 2017).
Seats like Ashfield and Bolsover have been gradually turning blue for years. As then-Labour MP for Ashfield, Gloria De Piero, put it in February 2019 (speaking about the loss of working class seats like Mansfield to the Tories in 2017):
Lazy thinking put the loss of these heartland seats down to Brexit or Jeremy Corbyn’s leadership, but the left’s decline with many traditional voters had been coming for some time.