I think replicants were programmed specifically not to live long, not that they couldn’t...right?
"The light that burns twice as bright burns half as long."
I was a little confused about this point in the movie. On the one hand they described the short life expectancy as a fail safe, but on the other Tyrell went through several reasons why they couldn't extend Roy's life - maybe they couldn't extend a replicant's life after it was created. But if that was the case, what was so special about Rachel?