People said the same thing about Aussie rules in the Northern States, gridiron supplanting baseball as the most popular sport in the US, ice hockey in the desert, the NFL building a significant audience outside of the US, soccer becoming a major professional sport in the US, etc, etc. Nothing is infallible, nor is anything is too big to fail, and something having never happened doesn't make it impossible.
If the NRL, or AFL for that matter, were smart about it and willing to do whatever was necessary, then there'd be little to stop them from taking a significant share of the NZ market given time. Both could offer an alternative local professional product and throw significant amounts of money and resources at the grassroots in a targeted way that the NZRU would struggle to compete with over an extended period.
Supplanting RU as the major sport in NZ would be an unrealistic goal, but taking somewhere between 10-20% of the market would definitely be achievable if they gave themselves a generation or two and a few hundred million dollars to play with.
For multiple reasons I doubt that such an attempt to grow in NZ will happen any time soon in either sport, but that doesn't mean that it couldn't, or that it'd be impossible.