Jamie Reed resigned precisely because his constituency was one of the last to be massively dependent on a single employer, making it wildly untypical, and especially difficult for Jeremy Corbyn to hold.
So the result at Copeland, the result that Reed wanted and even contrived, cannot be said to mean anything much beyond itself.
The real story is the extinction of UKIP at Stoke Central and, by extension, at large.
UKIP's voters have gone home to the Conservatives, such as to give the latter their majority at Copeland.
That majority has come, not from Labour, but from UKIP.