As to how many site visits these guys made, I think both Doak and Brad Klein have realized how few it was after researching the papers of Mac and Ross, respectively. Of course, each had good associates.
I find it a bit odd that many here would credit plaster models and good plans as a substitute for full time field supervision, when you give modern guys flak for computer drawings and any associate taking over some of the work. I suspect Ross sucess is in having his large crew of construction people (whether employed directly by him or not) responsible for most of his best courses.
For the busiest architects today, the model is about the same. Nicklaus and Fazio, have a long time associate on the project close to full time, much like Ross would. Even Dye does that, although he makes the Owner hire them, to avoid the business complications of a payroll, etc.
Another factor I haven't seen mentioned is the fact that perhaps, some of the greatness of those courses was added by time itself (maturing trees), and minor design tweaks by uncredited supers, etc.) Modern guys haven't had that luxury quite yet! For that matter, we haven't had the luxury of people pining for our "long lost style" yet. It will happen, just as it will happen even for courses designed in the supposed dark ages of the 50-60's.
As I have said before, I have seen Ross plans and sketches. I don't know as much about Flynn. However, plans today are much more detailed (with greens planned out to the 0.1 foot) than they ever did. And notes on Ross plans focused on where they would come up with dirt, approximate bunker depths, etc.
Could we conclude that except for a few rare cases, like Pinehurst, that those guys approached often their BUSINESS with a "just good enough" mentality, since the money was good for spending whether every course was a classic or not?
Just a thought......