> Though it can be argued that among social, intelligent creatures, living grandparents holds value for the species.

This is taken to an extreme in Larry Niven's "known space" setting. There's a bunch of interesting stuff I won't mention because spoilers, but imagine a very-humanoid race where the adults are not intelligent, but later they lose the ability to reproduce and gain hyper-intelligence, strength, and a ruthless drive to protect and advance their descendants.