Well, yes. Changing demography is a factor, as is differential turnout. There's certainly more to it than simple vote-switching.
But even so ....
I could have added NE Derbyshire, Middlesbrough S and Copeland to the three I mentioned (the comparison in the last-named being with the 2015 GE, not the intervening byelection).
And I'm also looking at the Tory near-misses in 2017: that is, the Labour seats that they came closest to capturing. If we confine ourselves only to the ones they missed by 2500 or fewer, we have (in ascending order): Dudley N, Newcastle-u-L, Barrow, Ashfield, Bishop Auckland, Penistone, Wrexham, Wakefield, Wolverhampton SW, Stoke N.
All I'm saying is that there's a pattern here.
I live in one of the Tory gains from Labour at the last GE, so have some idea of what I speak.
There is a pattern, its just not the extremely simplistic one that the likes of John Harris have dined out on.
Almost all the seats you list have either voted Tory in the past, or come close to doing so (Stoke N and Penistone being maybe the only exceptions)
It is also interesting that pundits tend to take far less interest in seats that are moving the other way. You previously mentioned perhaps the most spectacular example of this last year - Hove - upthread, but how many hacks have paid a visit to "understand" what happened there? All I can recall is a piece pre-GE by Nick Cohen, saying - with his now customary accuracy - that despite a dreadful Tory candidate Labour were likely to lose it, and that this was (surprise!) all Jeremy Corbyn's fault
Well, quite. It's two-way traffic and Labour's areas of increased strength are equally important and are the flip-side of the same trend.
The Brighton area has been a Tory horror-show in recent years and the remarkable 2017 result in next-door Worthing E suggests the contamination is spreading. Brighton's reputation as Lambeth-on-Sea is certainly being borne out electorally.
And this is my point really. Brighton, despite its location on the south coast, has quite a lot in common, culturally and demographically, with the sort of seats, balanced between inner London and the outer suburbs, and very sharply trending Labour, that got this discussion started.
I haven't drawn up a list of Labour's near-misses of Tory seats in 2017 but I'm sticking my neck out and guessing that it would have a strong bias toward the London area and the fringes of other major cities.