In the big picture, classic courses have basically been dropping out of the rotation forever, starting with Prestwick, and up through Merion (albeit making an encore) and Cherry Hills (due to elevation) and golf has survived so far. There are a bunch that were somewhere between one hit wonders and legitimate classic course venues.
Why would you think the current era would be any different?
Mark's baseball analogy is a good one. Isn't Fenway already on the road to the wrecker ball? And, I have to ask, what percentage of average fans prefer Wrigley (on a regular basis) over one of the new creature comfort filled stadiums, such as the ones I am staring out my office window at?
What % of fans will want an Open full of iron tee shots on a short, tight Merion course vs. seeing Phil and Tiger bomb drivers on a newer course?
Outside of golf and the well established trend for nostalgia, isn't American culture in general all about "ring out the old, ring in the new?"
I see the ramp up of using new courses tied with the 1997 ramp up of distance from the ProV1. The competition is the primary concern and the older courses were - and will continue to be - used when it was believed that they provided the best challenge and venue. Many can no longer do that, and more will hit that group, kind of like income tax bracket creep, over time.
Of course, things move slow and there are a variety of opinions, so the USGA and PGA are hedging their bets by mixing new and old. I bet that in a decade, the mix will go from 70 old-30 New to the other way around. IF there are fan/player complaints or logistical problems, then they may adjust the rotatations. In reality, its probably a constant adjustment for those organizations anyway.
But, eventually, I think playing the Open at say, Winged Foot will be done as a rarity and novelty as much as playing at Bethpage or Torrey is now.
Again, just MHO>