At what point in our history did green speeds over take importance of green interest?
Presumably it was when golfers started evincing a preference for putting on fast greens instead of slow ones.
You can make more 20-foot putts on a smooth, subtly-contoured green Stimping 12 than you can on a smooth, somewhat more boldly contoured green Stimping 9. Making 20-foot putts is fun and memorable.
Ask any golfer after he walks off the course about his round. He will quickly tell you about any and every putt over 15 feet that he happened to make that day. By contrast, doing a good job of two-putting wildly breaking putts on a severely contoured, slow-rolling green is fun but not as instantly memorable.
Good players (or good putters at least) are now addicted to making putts on fast, flattish greens. They are never going to choose to go back. Lesser players tend to take their cues from the better ones, often to their own detriment.
P.S. Or put another way, the great mass of golfers prefer to have their nerves and touch tested by very fast green speeds than having their stroke and imagination tested by bold contours.