All the best development systems in the world are pyramids (the football pyramids, collegiate system in America, take your pick of examples), larger at the base and smaller at the top.
By going down this route the NRL would effectively be creating a bottleneck that restricts the professional pathways in the southern hemisphere to the NRL clubs own systems and players that are poached from RU. In other words you'd have to be picked to join one of the 17, soon to be 18, NRL club systems development pathways to have any realistic chance of making the NRL, which will necessitate significantly restricting the talent pool and the amount of juniors that are exposed to elite development programs.
Arguably that'll be good for the NRL clubs in some regards, as there'll be less competition for the best players in the younger age groups, but it'll be a disaster for the sport as it'll restrict the amount of opportunities in the sport and you will see more kids with potential giving up the sport at a younger age simply because their isn't room for them in the system.
The idea is mid 20th century thinking, and we're not in the mid 20th century anymore.