"the older settlement has been left behind by a more prosperous settlement"
That's true.. But the way I'd interpret it is the opposite way round, that somewhere that we now consider civilised and busy was once just a furzy field. And that we should remember that civilised places can return to furzy fields. Stuff about the circling of history and suchlike? and that the people there were like us, rather than us being different and better.

Although the Saltash/Plymouth thing isn't quite the same because Saltash looks like it's still rather thriving.. so that's a different one.

I guess a handy saying like that easily gets passed around, like the variations in kids' rhymes in playgrounds according to region. I'm sure I've added a number of them but can't remember where they are (the search didn't turn up much).

A Derbyshire version...

"When Chesterfield was heath and broom,
Leash Fen was a market town,
Now Leash Fen is all heath and broom
And Chesterfield a market town"