They say that if one discussion is held for some time, Hitler pops up sooner or later.
The same seems to happen with Harley when a discussion about bikes is being held.... :-)
Now let's go to the extreme. All the CB750 has been dismantled and there are just two examples on the face on the earth, and both owners know each other.
One day, the engine of one blow completely, with a rod protruding out of the crankcase. He loves his bike and probably will try to convince the other owner to sell him the engine. How much should he have to offer to convince the other to sell him the engine? If he sells, he will have an unusable bike, so he would only sell the engine as long as the price is, at least, the price of the whole bike.
Then, it is either the man with the broken engine a) pays what the other guy is asking for his bike, b) sells the remains to the other owner or c) have the parts needed custom made. As long as option c) is cheaper than option a) -and chances are it will be-, the two bikes will remain on the road. If the chosen option is b), then the answer to the original question is that there will be parts bikes until the last bike vanishes. We are talking about parts bikes, not about the price of a parts bike. I bet that the price of the second-to-last CB750 ever would not be cheap, but there would be it.
As long as there has been parts bikes before, option c) is out of the question because it is always cheaper to use old parts. But if they doesn't exist, they can always be made. It is just an economic decision, whether the value -value, not cost- of the vehicle to the owner is higher than the cost of the repair.
There are not many Bugattis for parts, but I bet that if you have one, you can get any part you need....