"It is good that Corbyn keeps winning elections, because he can't win elections" is a line with a fairly obvious flaw in it.
The only places that he ever loses are places that have never voted Labour, anyway.
If UKIP cannot win Stoke Central, then it just cannot win.
Likewise, if the Conservatives cannot win Copeland, then they cannot win any seat that they do not currently hold.
In the Remain heartlands of the South, they are already on course to lose dozens of those seats to the Lib Dems, who need only to be the First Past the Post in them.
Meanwhile, if Labour does not lose either Stoke Central or Copeland, marginals that are used to right-wing MPs, then it is not going to lose anywhere in 2020.
That hung Parliament is going to be far more interesting that the last one.