Just look at how the athlete has evolved from then to now......the athletes today are bigger, stronger and faster. If you plucked the 100 best players from the 60's and 70's.......I'd bet only about half would even wind up starters in todays NFl.....and MAYBE 5-10 could still be Pro-Bowl type players.
Now, if these same players grew up in the same enviorment as today's NFL players did.....then I would think that they would still be greats.....because they'd have had the lifetime of training to make them equal physically.....and still have that something special that made them great in the first place.