Game changers...
Could it be much of this lies with changes over time in equipment?
Karsten Solheim, etc - perimerter weigheted clubs limiting shotmaking?
Wally Uihlein, etc - balls that spin less and go far?
Ely Callaway, etc - drivers that hit the ball a mile?
Tiger Woods, etc - how many wonderful courses have been "Tigerproofed"? Adding length.
Rainbird, etc - irrigation, grooming and expectations have gone crazy.
...among others
Put them all together...and viola!
In the last 20-30 years, all of the above have driven the arms race for longer courses, which obviously and naturally affected design. With these advancements, many great courses cannot be considered for Majors...simply due to length. Kind of like driving a 500Hp car on a gravel road.
Fact is, the vast majority of those who play have plenty of course at 6,500ish yards. Yet, we seldom see a new course at less than 7,000 yards.
Most here adore the old classics, and a good many of these aren't long. Coincidence?