There are, of course, massive stumbling blocks to this idea.
With the current Super Rugby system, our top 150+ professional rugby players are ALL exposed to the top level of domestic rugby, and with very rare exceptions, the All Blacks are chosen from that base of players. If we were to return to a 1990's style "South Pacific Championship" style of competition (which is what Kirwan's idea basically is) where only the top four or five Provincial sides make it to Super Rugby, then two things will happen;
1. The players who aren't playing for the top four or five sides will be denied access and exposure to Super Rugby. Imagine what would happen if Wellington and Otago were to fail to make it into Super Rugby? Absent from Super Rugby would be Beauden Barrett, TJ Perenara, Ardie Savea, Jordie Barrett, Ben Smith, Ngani Laumape, Ben Lam, Vaea Fifta, Dane Coles, Liam Coltman, Matt Faddes & Michael Collins... and many more.
2. Players will migrate to play for those top few sides, which will severely damage (and IMO, eventually kill off) rugby in the Provinces.
There is only one possible way I could see anything like this working.
1. At the end of the National Provincial Championship, the All Black selectors would choose our top 150 players from all Provinces
2. Any players in the 150 who were not in the squads if the top four/five teams that made it so Super Rugby will go into a draft.
3. The five teams selectors pick players from the draft to add to their squads, lowest ranked team gets first pick etc.
This way, no matter which five teams get to Super Rugby, all of our top players will play.