That's one of my theories. It's already starting to happen. Soon the average Joe won't be able to afford "vintage" and the next step will be the 70s, then the 80s, etc...
Right now the 50s are through the roof. See the 55 Strat on ebay at 46Gs and counting for evidence.
http://tinyurl.com/8mtrt
The 60s are hot now. The 70s are starting to come on, too. My 74 Tele DeLuxe is in the 2 grand range now. Hell, my 96 Strat is worth a few dollars more than what I paid for it already.
So, in answer to the question "When does the turnover take place?,"
it seems like it will be in the next ten years (30 year interval).
Remember when we were kids? The word "vintage" didn't exist! There was no such thing as a "vintage guitar." They were just "used."
Damn. If I'd only known then what I know now .....
