I think high volume production helps make something unexpected a classic because:
a) There are so many of them the vast majority of owners will never invest in serious repairs once the car gets older. It will just get towed off to the scrappy.
b) There's more people who had one when they were younger, or whose dad had one, or a friend, or whatever and who now have disposable income and fond memories of the car.
c) Large runs should, in theory at least, help parts supply. Making them easier classics to look after.
Twenty years ago I was a big fan of Capris and you could, quite literally, pick up a copy of Autotrader and find dozens of the things for under a grand, even including the 2.8i. Now there's nothing worth having under £5k, and they're not even particularly good cars, in an objective sense. Most of them are under-powered, they handle like a drunk pig on a tea-trolley, they've nearly always got tin-worm, they drink petrol and they're more than a bit plastic-fantastic inside.
The same sort of thing is true for Cortinas, Mk I and II Escorts, even the Fiesta. Huge production numbers haven't done their current day values any harm at all.