I suspect that if alive today, Leonardo and others might have used computer graphics to assist in the planning, studies or other aspects of their work. But I suspect that their work would not necessarily have been the same, given the same subject matter, and that I might not necessarily have the same high regard for them and their work.
I've contemplated the use of computers myself for these kinds of preparatory stages and I think that it may work for some processes, nevertheless, I am committed to the use of hand-eye-mind coordination of painting that I have always admired over the more "mechanical" means of creating visuals. There is something in the "human element" of painting that I seem to need if I can accomplish it myself. This is one reason why I am not a photographer.
Richard