This is a question of logic that, if it had been considered during the story input stage, Dramatica could possibly have drawn attention to, as a fundamental anomaly.
Given that the Nexus 6 Replicants were able to have memories implanted, is it not reasonable to suppose that the memories and experiences acquired by the replicants, could simply have been transferred to new Replicants, so that they would, in principle, be immortal.
That idea is explored in a version of the script. After Batty kills Tyrell, he stumbles into a room and finds clones of Tyrell waiting in stasis. Why this didn’t make it into the final movie is a mystery to many.
According to the commentary on my “Final Cut” DVD, there is hours of footage that never saw the light of day. Perhaps, like the Kubrick footage that was tacked on to the original release, some of that missing footage will turn up in someone else’s movie 
I suppose that what we know of as the movie, regardless of the director cut variations, is the least worst option via which the fewest of the elephant traps are evident.