The salary cap is not RL in the UK's problem, from what I have read it's that most clubs have been poorly run and that without the salary cap things would be much worse. I could also blame the administrative bodies but there's just not enough money to go around because it's a regional code in the NH. I guess you could argue the NRL doesnt do enough internationally.
The NRL has even a larger safety net and just as little parity, because the best young players want to play for the best and largest clubs. Even in systems designed for parity like American sports this happens; especially for neutrals, there is still the appeal of watching the best go at it.