I'm saying that the game has not changed significantly since the 1970s. The only big changes are professionalism, full time player commitment, strength, fitness and speed improvements. Players are built bigger, stronger, faster, which wasn't possible back in the 70s as much because players often had two jobs (a difference). Seems I give examples of similar roles played by players forty years ago, yet the implication is that, without evidence, that there have been significant changes in the roles played. The differences depend on who is coaching and the specific talents of the players.
There are no Menzies today not because the game has changed but because there are no Menzies. If Menzies was playing today and had Cliffy inside him, you'd be seeing much the same thing that happened during their career. What changes is specific (not general) strategies by coaches. And that can depend on the quality of player in their stable. But strategies get eventually countered and new strategies are introduced. And if you look closely, often the new strategies are old ones revisited. For example, the alleged delaying tactics suggested to be in the Storms style (hold onto tackled player longer etc). Watch the 1969 grand final again Balmain Vs Souths, Same thing was happening. A tactic revisited.
As for mobility being better in the back row today I would suggest you consider Menzies, Gartner, Kosef. All very mobile back rowers in the one team. Watmough, Price, Phil Lowe (1960s), Coote, MacCarthy, Clyde, Mackay, Roberts ..I could go on, all very mobile. Just depends on the player's skill. An old timer would suggest Raper was one of the most mobile payers in the game. They maybe fitter today but no more talented and mobile.