I don't think this will hamper young Aussies. I mean they're making an extra roster spot for these Next Gen kids, and there's no saying that some of the Next Gen kids won't be Australian.
Furthermore, as always, it's up to the teams who they pick up and who gets court time and who doesn't. I think you could make a case for at least one player on every NBL team who shouldn't really be in the league - Brendan Teys, Kevin White, Greg Hire, etc. If one of those guys lose a spot to a likely draft pick that will help raise the profile of the league, is Australian basketball really losing?
For as long as there are players who make you cringe to see come on court in our league, I think we have plenty of space for extra imports, especially with new teams expected to be announced for next season.