So I left Synth Shaun in the Institute to be blown up with the rest. My decision was in part due to my understanding (or misunderstanding) of how 3rd/4th gen synths work. They don't age. is this correct? If so then Synth Shaun WAS an abomination. I mean imagine if I had taken him back to Diamond City with Piper and I? He likely would befriend Nat....and then eventually Nat would grow up....and he would NOT. Forever a "10 year old boy" with all the people in his life (except for Uncle Nick Valentine)....growing older and older....and dying.
If I understand the tech right (and maybe I don't!) THAT is the true tragic legacy and evil of the Institute!