Charlie,
You don't see players rotating today?
You don't think Adam Scott could hit a 1960's 3 iron?
Interesting because I have one with the exact same shaft I use in my irons today and I don't hit it any different than my 4 iron (it is 4 iron length and loft)
I'd go with a different theory.
Put 10,000,000 athletes in a a room and have many/most get plenty of time,supervision and training and 200 great ballstrikers emerge.
go back 50,60,70 years and put 500,000 kids who weren't great at other sports (with multiple exceptions) in a room, have them caddie much of the time, drink, gamble, and or work part time and less good ballstrikers emerge.
I simply found it interesting that you chose the word x-factor, as Jim McLean used that word to describe coiling shoulders against hips on the backswing to store maximum energy on the backswing, via increased downswing rotation.
Ben Hogan specifically writes about this(restricting the hip tun) in his book, and he is one of the examples McLean uses in his teaching.
There is no doubt a lot of nonsense in today's teaching world, and no doubt a lot of opportunistic quacks, but on balance, instruction is light years better than 50 years ago.
In fact the 1960's (particularly Johnny Miller, Jack Nicklaus era) were the dark ages of golf instruction as both greatly influenced teachers to stress lateral motion on the downswing to a fault. "drive your knees to the target"resulting in a "reverse c"
The 1970's were the era of short careers as players employing that method had bad backs and duck hooks
fortunately for Miller he didn't keep the club "down the line" as he advocates ad nauseum on TV, but rather rotated hard allowing the club to swing left, keeping the club beautifully on plane.
Nicklaus, unlike most players of his era, was a great athlete, and used his power to great advantage.
Nowadays he would simply be one of the great athletes out there, although he would hold his own via his stronger mind.