Lets get a few things right.
There is no such sport as "NRL". The name is Rugby League.
Re Wingers 'doing very little", NFL is the only sport where wingers get interchanged! In fact, a change of posession means that an entire team gets interchanged. Now, if posession changes are similar to League (around 70 posession changes per match), that means EVERY player gets interchanged 70 times!!! Where is the endurance in that???
NFL is the football world'sversion of Cricket. Cricket, too, is exciting, seeing a pace bowler thundering in from the outer gruond, delivering a lethal ball to the batsman, who dances down the ground and belts the thing over mid-on for a boundary. Glorious, rivetting stuff. It's just that the next 5 minutes where every member of the bowlers team picks up the ball, has a look at it, throws it to his mate, through 11 pairs of hands until it reaches the bowler, who walks slowly back to the fence....zzzzzzzzzzz.
In Australia, the 4 codes of football that we are familiar with don't have such frequent stoppages. Even poor old Union, the domain of the fat football player, is based on a theory of keeping the ball in play.
As for multiskilling, forget it. I recall (name forgotten) an ex AFL player who played gridiron in the US. His job? To kick a ball 50m in 4.6 seconds. 4.7 would be a failure. Had to be 4.6. Imagine Andrew Johns having nothing else to do but put an occasional kick with specific measurements! The amazing thing about the Johns brothers was their ability to pull a kick up a few inches short of the dead ballline regardless of WHERE they kicked it from. That level of brilliance is never used un NFL.
Pre interchange, there were only 2 replacements allowed during a game. These players had to play an entire reserve grade match to qualify for their reserve status. And before that, there were no reserves at all. League folklore is full of men like Eric Weissel, Alan Prescott, John Sattler etc who have played hugely improatnt matches with broken bones. Weissel once set up an Ashes victory with a 75m run on a broken ankle. Gridoron's answer to this is......?
The population difference is very crucial. With 15 times the population, you would expect the basic athlete, upon entry into the Gridiron big time, to be bigger, stronger and faster than his League counterpart. Yes, the genetic fetures of African ancestry means a pool of large people. No matter. While there are (estimated) 70 million African Americans that can provide the NFL with their giants, we have a few hundred thousand Maori, Polynesian, and Koori people that can provide our code with scary, tough, ruthless people aka Quentin Pongia, Kevin Tamati, Gordon Tallis, Mal Merninga, Carl Webb, Iafeta Pa'aleasiina etc.
And the final nail in the Gridiron coffin - apart from the USA - the land of sporting isolationism - has been adopted by exactly ZERO countries,despite over a century of hype. Maybe other countries can't get the gear - it's all bought by motorcycle riders and bedding manufacturers.
But, in the end, who are we to argue? I'd prefer to leave the final word to one Nate Turner - boom young gridiron potential superstar - who decided that all the fame, fortune and glory of playing Gridiron was no match for the utter joy of playing Rugby League. And he got to represent his country, which is more than any Gridiron player has ever achieved.