This theory used to hold weight, but after seeing Ireland turn into a rugby powerhouse that beats NZ & South Africa regularly, has to be refuted. Irish rugby is built on a small catchment of Dublin schools. Way smaller than Australia’s. Ireland have become world No.1 multiple times last decade. Australia’s issues seem to be with system & high performance. Players go to Europe and get better all the time. Wallabies could get to No.1 again with AFL and NRL just fine. Conversely, a world with no AFL and NRL wouldn’t guarantee anything. NRL players, by and large, can’t tackle well. AFL players can’t do it at all.