Sounds like you think you do..
You sound as if you believe you're the only person that has ever played the game, and are uniquely qualified to talk about it, and others who feel differently to you, are not.
Which I can assure you, isn't the case. I'd say a vast majority of posters (i.e. over 90%) on this forum are quite knowledgeable in terms of the game (granted they may have missed an edition or two of RLW though) and understand full well what it takes to play the game.
Hayne is a professional athlete, If he can't handle defending maybe he shouldn't be one at all then. I severely doubt Hayne would have any trouble defending, especially now, he is physically a lot different now than he was when he left us in 2014, and may actually be more suited to a halves role now than he was before, and inversely, less suited to a fullback role. But then, I doubt he'd have any trouble with either position IMO.