There are too many variables if Nate loses. Conor will fight Eddie, then if he loses, he'll beg for that rematch, if he wins, he'll fight some other guy and maybe lose that, then beg for a rematch, etc. Nate will get sidelined. If Nate wins, he's a big name and gets another big payday with whoever he fights next, and if Conor goes back to winning, he'll probably want yet another crack at Nate for another big payday. There's nothing better for Nate's career than to keep winning.