Honestly, I think half the reason people complain here is that deep down they know that most of the locations for expansion are actually going to work long term, just with varying degrees of success. They're just worried that their preferred location won't be one of the next 3 chosen.
But the NRL isn't restricted by playing talent numbers like the AFL is. Our player talent pool is basically wherever rugby union is played. As the salary cap increases, it becomes a strong magnet for talent attraction, converting players who would ordinarily go to Super Rugby, and Union players overseas. On top of that PNG is barely tapped, NZ's and the Pacific Island numbers will increase as league increases its popularity and subsumes Union in the region, and the WA/VIC will start producing players in bigger numbers over time. In comparison, the AFL in 20 years time will still get 99.99% of its players from Australia.
There's no reason why a 20 team league is the end of expansion. Perth, Adelaide, a 5th Queensland team, New Zealand 2 and PNG can all happen over the next two decades with proper planning. A 22 team league is manageable.
The saturation in Australia would be the same/similar as AFL's (19 NRL vs 19-20 AFL teams) but also spread out in more cities (10 vs 8). Plus on top of that there's be 3 teams across NZ & PNG (another 16-22 million people).
If that's not a blueprint for being the most dominant sport in the whole region, I don't know what is.