Mike,
Interesting topic. Part of me wants to say that old-guy swings generated a lot of power, too, and therefore required a tremendous amount of "work" in the technical / physics sense. In other words, they put just as much pressure on their backs. Golfers throughout history have struggled with back pain so perhaps the "new" swings will tip a few more in that direction. But where's the evidence? Are we just forgetting the old-swing induced back problems?
For every Tiger there's Annika Sorenstam, Fred Couples, or Jay Haas. Oh, Jack Nicklaus, too, remember? Back in the middle of his career?
EDIT: just remembered another -- Ballesteros.