Interesting discussion with a lot of good points, but I think two huge changes during the 20th century are very critical to the cause/start of the "dark ages" and the end of the "dark ages".
The first relates to available land. Back in 1900-1930 for the most part there was ample availability of high quality land reasonably close to major cities (or accessible by commuter trains in the large cities such as Chicago and NYC). Ever notice that almost all great classic courses hav e train tacks near them...that was the only way to get there from offices in many large cities.
Great land is a huge plus IMO for great golf courses. From 1930-41 few projects commenced (outside of a few funded by gov't entities such as Bethpage) due to the depression...one exception was Old Town Club in Winston Salem NC...that was funded by $$ (and land) from the Reynolds family (Reynolds Tobacco). Interesting that people continued to have tobacco products on their list of necessities during the depression.
After WWII there was a huge push to build housing for the returning armed forces and that goggled up large portions of available land near major cities...and drove up real estate taxes. Think of some of the courses that closed or moved during that post war period...Fresh Meadow (NY), Pomonok (NY), Englewood (NJ) (I grew up in NY so know these NY examples). So the architects were left w so-so land...the great land on the north shore of LI, Westchester, near Lake Michigan in Chicago, or in SW San Francisco was no longer available or way too expensive (or both).
Then in the early 90's Dick Youngscap started working his long time dream to build a course in the "impossible to get to" Nebraska Sandhills. In so so many ways perfect land for a golf course...fabulous land movement, just about unlimited perfect water underneath, sitting on hundreds of feet of perfect sand, etc. But if you build it "would they come"? Youngscap and Coore/Crenshaw took a huge risk (in retrospect looks like a no brainer...but at the time risk was major...just like buying shares of Apple in 1995 looks obvious in retrospect). They did come...in droves...club membership sold out quickly and that said to Mike Keiser go ahead with that out of the way site along the Pacific Coast in OR...which also was a huge risk. Those two developments ended the "dark ages" because they opened upon for development at lots of out of the way sites and led the way for great new architecture and courses (based on the assumption that great architecture is substantially easier on great land). BTW...that s=does not mean great architecture is easy in those cases...but easier.
As a side note regarding the above, IMO both Youngscap and Keiser deserve entry into the Golf Hall of Fame for ending the dark ages and opening up the 2nd Golden Age. Sandhills and Bandon may not have been sufficient for the 2nd Golden Age but no question they were necessary.
OK...now the second huge change...environmental laws and regulations. This is NOT meant to get into a political debate about the costs and benefits of these laws and regs. But they did become a real complicating factor in terms of finding usable land for golf courses. There is no way that tracks such as NGLA, Shinnecock, ANGC, Seminole, Shoreacres, Merion, Lido etc etc etc could have been build if today's laws/regs were in place back then. Over time architects, construction folks, lawyers etc found ways to more easy deal w the regs...but that learning process took a while and contributed to the dark ages as well.