Well, I'm not sure what golden age of teaching in the universities you are thinking of, but my undergrad days were half a century ago and I can count the inspired teachers on the fingers of one hand...and have plenty left over.
I said nothing about a "golden age of teaching." No doubt that inspired teachers have always been rare. The difference is that now inspired teaching is not rewarded at many universities.
There was a time, before the Second World War, when science and engineering professors at most universities were expected to teach three or four courses a semester. Now a teaching load of two courses a semester is considered heavy.
As teaching loads decreased, enrollments increased. How have the universities managed? One way has been to increase the class size; another has been to hire graduate students and adjunct faculty to teach.
I have observed these developments firsthand. When I was on the faculty of a large Midwestern university, I used to teach a course that enrolled 800 to 1200 students every fall. One department head at the same university bragged that 85% of the undergraduate student contact hours were taught by people other than regular faculty.