Based on my personal experience with hickory golf, 85% is a good scale. ie a 6,000 yard layout instead of 7,000 yards still feels like the same sport once you mentally adjust. At that distance, you're right back in the sweet spot of golden age distances and you eliminate walk backs on almost all courses (classic and modern).
And in many ways, the shorter course with the same dynamic has a better overall feel to it, especially if the conditions allow for some roll out. The fairways become effectively wider from shorter tees and so it makes playing for angles more achievable. All of this would be pretty silly with 460cc drivers though.
But I agree with Kalen in that it's a small percentage of the overall golfing population that hits it too far for a 6,000 yard course even with the best of technology. A square path with a persimmon outdrives an over the top hack with an M2 every time.
Even most mid level amateur tournaments- like state am qualifiers where the average handicap is only a few over scratch, are played on courses around 6,700 yards and very few break par. As a specific example, take Kankakee Elks (a Langford course that is close to it's original shape at 6,596 yards) in a qualifier for last year's IL state am. In a field of 44 players, the lowest score was -1 and the field average was +7.3. There were 2 eagles, 90 birdies, 240 bogeys, 61 doubles, and 16 dreaded others. One of the 2 players who led the field had no birdies on the par 5s. And while this field isn't a US Am field, it's still players who are at a level that 95% of golfers can only dream of.