Citroën metropolis concept car (2010)

It wouldn't. Compare Chevrolet volt concept car with actual Chevrolet volt car.

The Citroen SM, which was essentially a concept that made it into production, is probably my favourite car design of all time.

are they? It's all the same company with the brand today just used for appealing to localized government funding (because we can just shutter that factory you have there!). In the small marketshare electric future these multiple brands will be useful to kill one by one without tarnishing the remaining brands too much.