I think its the "signature aspect" of the firms you are thinking of that wasn't as present in the Golden Age. At one time, and I may be wrong, JN contracts stated he wouldn't do another course within X miles or years to guarantee the value of his signature. As he got busier, he realized the folly of that and stopped that clause.
Travel made most golf architecture regional in those days. Besides the practical aspects of a close gca, travel and a smaller game made the original golf circles quite small. Its possible that Tillie attended every golf related event in the NY metro area and had a better chance of selling every course design than any competitor. Now, you wouldn't likely find Mike Keiser and Donald Trump in the same room.
Also, in the early days of ASGCA, meeting minutes suggest that each member was "protecting" his territory and respecting those of others. While the biggest names of the Golden Age were certainly national and international in scope, I believe RTJ used the 707 to break down any travel barriers that existed, or any gentlemans agreements. Now, anything goes, including everyone working everywhere, and one guy working up and down the street.
How many do the Florida guys have up and down the coast? How many does Fazio have in Dallas now? (3) How many does Palmer have in Palm Springs? Heck, even I have three within fifty miles in Minnesota, and a bakers dozen here in DFW, with two more on the way.
The problem for the gca is the clients wondering if you gave them equal efforts and equal quality courses. I was lucky that all three of my MN courses got recogition, including consecutive GD back to back best new,so neither feels slighted. However, I doubt that human nature has changed that much. How do Scarsdale members feel about WF, for instance?