For every PI player RU loses there's a Saffa, European, and increasingly people from other regions, patiently waiting to take that opportunity. Would losing a bunch of PI talent hurt the average standard of RU for a while; maybe. Would the impact of that be felt in any significant way outside of Pacific region; with the possible exceptions of Japan and France, and even in those examples it'd only be fleeting, almost certainly not.
Even if the NRL does 'conquer' (for lack of a better term) the South Pacific, which is still a big if, it will still only be a big fish in a small pond. While RL is bragging about conquering a bunch of impoverished, tiny, third world countries (not having a go, just speaking frankly about the situation), RU will still be making moves and growing in most of the largest markets in the world.
So really what's the big deal? You'll have done the equivalent of giving RU a paper cut, it might sting for a second or two, but it won't do them any real damage.
If the goal is to compete with RU on the international stage, or even just to give them a good kick in the dick, then RL should look to grow the sport in the parts of Europe, Asia, and the Americas that don't traditionally have strong cultures of Union, and particularly the Indian subcontinent and South East Asia.
How you'd go about that in reality is extremely debatable (whether it's realistically possible at all is debatable in some of those cases IMO). But you'd basically be placing yourself in the position of filling the niches in the global market that they haven't yet, but undoubtably plan to in the future. From there you'd build RL's international product to a size where it's competitive with RU's, and must be taken seriously, without having to fight them for every inch at the grassroots level in every nation.
From that point you could leverage that growth to push growth in other nations, but we're getting way ahead of ourselves at that point.