I can see that but also the alternative. The modern forwards are as fit as hell as you say but for the modern game. Remember the drill the Canterbury did last year as punishment? That was a training drill in the 60’s a totally different type of fitness was required - forwards for example needed physical strength to endure the scrum contest but a lot less fast endurance but were not being able to be replaced. Modern forwards have never packed a scrum like the 60’s and would struggle with the strength, endurance and the continual contest. Ball security was much more important.
As I said a different game to today