When you look at teams that become premiers they have one thing in common, a blend of developed juniors and players recruited from other clubs. If you want an example, look at the famed St George Dragons during their 11 year premiership run. They had a good core of juniors but they had players they recruited from outside the club (e.g. Kevin Ryan who they recruited from Brisbane Brothers, John Raper who was recruited from the Newtown Jets).
Main reason this happens is that no matter how good your development/coaching paths, you rarely get sufficient quality juniors coming through which enable you to fill the entire team. Take the Parramatta Eels of the late 1990s-early 2000s. As they'd had a generation that spawned Grothe, Kenny, Ella (amongst others) locally, they thought they could replenish their side by solely recruiting from within. problem is the players who came up were not as good, or were not properly identified by the club and were recruited by other clubs. Hence Parramatta spent years as also rans down near the foot of the table during this period.
Parramatta only got back on the winners board in a serious way after the recruitment of Jarrod McCracken, Dean Pay, Jim Dymock and Jason Smith in 1996 as they filled gaps in the quality that was partly missing from their team.