I concur about people wanting their shiny new vehicle not to look old fashioned. My Land Rover was first registered in 1969 and still has its original 'G' registration plate in yellow, white on the front.
My 1974 CB500 can now have a black plate but I can't be bothered and it looks wrong. My 1972 CB350 has a black plate which I made myself in the 80s - no longer legal. When it goes for the MOT in a few weeks I think I will go for black plate but then the whole bike is very ratty and looks much older than it is really.
If you want to be authentic in every detail, none of the bikes on this forum should have black plates and personally, whatever age a bike is, I would not have a front number plate.