Kid doesn't need to beat Shanks for the new generation as a whole to succeed. He and Law might be relevant later in another context, plus you have Luffy (Joyboy), Zoro as his right hand man and Blackbeard.
After succeeding in beating Big Mom in Onigashima, there is little reason for them to reappear as main antagonists to Luffy as compared to Blackbeard and Shanks.