Good question indeed---I am 58 years old, and I remember what hotrods were like 40 years ago---not because there wert a high concentration of hotrods where I grew up in Ontario, but because I lived for the magazines out of California, which was and is considered the mecca for all things hotrod. Model A's were cheap and plentifull, and the junkyards were full of parts from the early 30's to the mid 50's which were basically "bolt on" to early Ford hotrods. We all thought that Ed Roth was a God, but none of us aspired to actually build cars like Ed was doing. None of us had money, and it was very easy to get what you wanted cheap at the boneyard. My mother who lived through the great depression is still alive, and she still pinches pennies like everyone who lived thru the great depression did, even though she doesn't have to now. Us older rodders are a lot like her----even when aftermarket parts became commonplace, and our bank accounts grew to the point where we could afford to buy them, we still went to the junkyards for our parts. Now, there simply are no junkyards for a lot of the stuff to build rods-----the oldest car you find in a junkyard here is 10 years, if your lucky---anything earlier is crushed and shipped. So----wanting to actually drive the rod we are building in this lifetime, we buy aftermarket.
The new guys coming into rodding now, have no access to the junkyards that us geezers did. With them its either buy from the aftermarket or don't get involved in building traditional hotrods!!! The junkyards are full of "ricers", the hotrod movies are about "ricers", and like it or not, the ricers of today are what the model A's were to us kids in high school back in 61 and 62.
Somehow, the act of digging around in a junkyard to get used parts has became "socially unacceptable". People don't trash hotrodders like they once did, especially if the hotrodders are buying shiny new parts for their cars.
Of course the aftermarket shapes the design of todays hotrodding. Mainly because other than us remaining geezers who still carry the torch, there is nothing else to shape it.