Nah, there's heaps of NZ players not coming through the Warriors system, and there's definitely enough for another team (the Storm used to carry half a team on their own). Many NZers go as teens though as you note. The Warriors haven't always been the best at predicting the future (the Bromwichs, SKD etc overlooked), so there's definitely an opportunity for the other NZ team to get players.
But the pool of players will decrease if the domestic game isn't sorted - for example, as I noted the comp is dead in my province, will a Tohu Harris of the future switch to NRL if he didn't grow up playing on Sunday. Also, there were lots of players going to league during U20s days as they could get money, but today most of the rugby Academies combined with NPC salaries will see young guys stay, or opt (like Eten Nanai-Seturo) for rugby.