I think NZ are authoring their own decline with their protectionist attitudes towards Super Rugby. As the league diminishes, so do revenues, then you have more players leaving NZ and becoming ineligible for ABs duty, along with a lower standard of play in Super Rugby.
If 25 years from now half of potential ABs are in Japan/France because Super Rugby doesn't make enough, even with top ups for national duty, they'll have themselves to blame.
Super Rugby is idiotic in its current state. You don't even need to watch to know the outcome of each game, even without following the league closely. People want hope for their side and to see competitive matches.
They need to share players across the competition. I doubt Callum Clarke or Beauden Barrett are suddenly going to forget how to play Rugby if they suit up for the Tahs, despite whatever nonsense spewed by Kiwis.
Ensure a certain minimum number of spots for Aussies/Kiwis in the entire competition, allow a certain few marquee players to stay in Aussie/NZ, then let clubs sign any player or even have a draft.
Works brilliantly in North America and we have the highest revenue generating sports leagues on Earth, soccer aside.
Ice hockey players get paid $10m/year. Even a 3rd tier American football league in Canada, with only 7 teams, gets crowds of 20,000+ people.
There's no reason that Super Rugby couldn't be far more compelling as a product and get crowds of 10,000 - 20,000 for each game. Even with smaller populations in NZ/Aus.