The D err... G-League is run and operated by the NBA and most of its teams. The league is now in a position where it can offer kids an option outside of the NCAA, controlling salaries, Development and all revenues on game day. The league is a new revenue stream, and penetrates the league into more communities. It’s a major success.
The NFL is big enough to essentially eliminate or limit any start up, should they cut into the NFL’s audience, profits or exposure. The XFL and other new spring league(s) have a chance to become slightly relevant, if the NFL looses a season. However, it’s a small chance. Currently, the NFL doesn’t need to protect itself from any other start up, so they’ll probably just see how well the new leagues are received, and act accordingly.